Nitride semiconductor crystal producing method, nitride semiconductor epitaxial wafer, and nitride semiconductor freestanding substrate

ABSTRACT

A nitride semiconductor crystal producing method, a nitride semiconductor epitaxial wafer, and a nitride semiconductor freestanding substrate, by which it is possible to suppress the occurrence of cracking in the nitride semiconductor crystal and to ensure the enhancement of the yield of the nitride semiconductor crystal. The nitride semiconductor crystal producing method includes growing a nitride semiconductor crystal over a seed crystal substrate, while applying an etching action to an outer end of the seed crystal substrate during the growing of the nitride semiconductor crystal.

The present application is based on Japanese patent application No.2011-198104 filed on Sep. 12, 2011 and Japanese patent application No.2012-197038 filed on Sep. 7, 2012, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nitride semiconductor crystal producingmethod, a nitride semiconductor epitaxial wafer, and a nitridesemiconductor freestanding substrate.

2. Description of the Related Art

For GaN-based material growth, a vapor phase growth method, such asmetal organic vapor phase epitaxy (MOVPE) or hydride vapor phase epitaxy(HVPE), is mainly used. In these growths, a seed crystal substrate, suchas a sapphire substrate, SiC substrate, or nitride semiconductorsubstrate, is installed on a holder, a tray or the like, and a rawmaterial gas is fed to the seed crystal substrate, to grow a nitridesemiconductor. Specifically, in the metal organic vapor phase epitaxy,the nitride semiconductor is grown over the seed crystal substrate, byfeeding an organometallic gas, such as trimethyl gallium (TMG),trimethyl aluminum (TMA) or the like, and ammonia. Also, in the hydridevapor phase epitaxy, the nitride semiconductor is grown over the seedcrystal substrate, by feeding a group III raw material gas, such asgallium chloride (GaCl) gas, aluminum chloride (AlCl, AlCl₃) or thelike, and ammonia to the seed crystal substrate installed on the holder,the tray or the like.

As the seed crystal substrate, a heterogeneous substrate made ofsapphire, SiC, Si or the like, or a nitride semiconductor freestandingsubstrate made of GaN, AlN or the like is used. Over these seed crystalsubstrates, crystal growth is performed so that typically a group IIIpolar c-face such as a Ga face is a front side, and that surface is usedfor device formation.

A problem arises in the nitride semiconductor crystal growth: When thegrown layer of the nitride semiconductor becomes thick, cracking tendsto occur in the grown layer.

In particular, in the thin film growth over the heterogeneous substrate,this problem arises when the thermal expansion coefficients of the seedcrystal substrate and the nitride semiconductor layer grown thereoverare significantly different. For example, in the growth of the GaN layerover the sapphire substrate, cracking tends to occur when the thicknessof the GaN layer exceeds 5 to 6 μm. In the growth over the SiC substrateor over a Si substrate, even the thinner GaN layer of the order of 2 to3 μm is likely to crack unless a complicated stress relaxation layer orthe like is built therein. Such a limit to the growable GaN layerthickness causes such various inconveniences that types of applicabledevices are limited, or enhancement of device properties is prevented.

The cracking mechanism in the thin film growth over the heterogeneoussubstrate is typically described as follows: Even when the GaN layer isgrown over the heterogeneous substrate at a growth temperature (up to1000 degrees Celsius), no significant stress occurs during the growth.After the growth, however, returning the temperature of the nitridesemiconductor epitaxial wafer formed with the GaN layer over theheterogeneous substrate to room temperature causes stress due to adifference between thermal expansion coefficients, leading to bimetallicwarping of the nitride semiconductor epitaxial wafer. In the GaN layerover the sapphire substrate, at thicknesses exceeding 5 to 6 μm, thestress exceeds the critical value, causing the GaN layer to crack.Herein, the critical thickness value (critical film thickness) of theGaN layer over the sapphire substrate is assumed to be 5 to 6 μm, but inpractice that critical value varies according to growing apparatus used,growth conditions, etc., and as it stands, it is difficult to determinethe definite growth thickness at which cracking occurs.

As well as cracking during thin film growth on the order of a few μmthickness described above, the occurrence of cracking has been a majorproblem even in the growth of the nitride semiconductor freestandingsubstrate having a few 100 μm to a few mm thickness.

There are two ways to fabricate the nitride semiconductor freestandingsubstrate: One way is to use the heterogeneous substrate as the seedcrystal as in the case of the above-described thin film, and another wayis to use the nitride semiconductor freestanding substrate as the seedcrystal.

In the freestanding substrate production onto the heterogeneoussubstrate, the use of for example, a Void-Assisted Separation (VAS)method disclosed by Patent document 3 allows a void layer to mitigatethe stress between the heterogeneous substrate and the nitridesemiconductor layer grown thereover. For this reason, the occurrence ofcracking due to the stress between the heterogeneous substrate and thenitride semiconductor layer as in the above described case of thin filmgrowth is suppressed, and the nitride semiconductor layer having a filmthickness of the order of 100 μm can be grown without cracking. However,even in this case, the situation at the initiation of growth is the sameas in the case of thin film growth: The growth on a face tilted from thec-face occurs around an outer end from the beginning of the growth. Thisleads to the occurrence of stress. At thicknesses of the nitridesemiconductor layer exceeding 100 μm, cracking tends to occur during thegrowing and cooling. Since the general thickness of the semiconductorwafer is required to be on the order of 400 μm to 1 mm from the point ofview of ease of handling, it is necessary to grow such a very thicknitride semiconductor layer in the growth of the nitride semiconductorfreestanding substrate as well. For this reason, the occurrence ofcracking due to the presence of stress in the outer end is a seriousproblem which excessively reduces yield in the production of the nitridesemiconductor freestanding substrate.

In order to remove a nitride semiconductor polycrystal adhering to aninner wall of a reactor or around a susceptor to place an underlyingsubstrate, Patent document 1 (JP-A-2007-320811) has disclosed a methodby introducing an etching gas into the reactor after growth, so thatnitride semiconductor substrate cracking during cooling and damage tothe inner wall of the reactor are suppressed. When growing the thicknitride semiconductor layer, the method of Patent document 1 has failedto reduce the occurrence of cracking during the growth, and has failedto increase the yield of the nitride semiconductor crystal, and has madesmall the obtainable area of the nitride semiconductor substrate due toan excess of the undesirable nitride semiconductor polycrystal aroundthe susceptor. Also, Patent document 2 (U.S. Pat. No. 6,632,725) hasdisclosed a method by feeding HCl (hydrogen chloride gas) into a reactoras an etching gas during GaN growth, to reduce GaN adhering to an innerwall of the reactor. Further, in order to reduce warpage of a laminatewith a GaN layer grown over a sapphire substrate, Patent document 4(JP-A-2007-106667) has disclosed a method by nitriding and hydrogenchloride gas etching of the sapphire substrate, to form an unevenstructure of an aluminum nitride in the surface of the sapphiresubstrate, so that GaN is grown over the sapphire substrate having thatuneven structure of the aluminum nitride.

-   Patent Document 1: JP-A-2007-320811-   Patent Document 2: U.S. Pat. No. 6,632,725-   Patent Document 3: JP-A-2004-039810-   Patent Document 4: JP-A-2007-106667

SUMMARY OF THE INVENTION

One of the reasons for it being difficult to determine that criticalfilm thickness of the GaN thin film over the heterogeneous substrate isdue to an end face effect.

Typically, in device fabrication, a Ga polar c-face is used. As shown inFIG. 1, the substantially entire surface of an epitaxial wafer formedwith a nitride semiconductor over a heterogeneous seed crystal substrate1 is covered with a nitride semiconductor crystal 2 a grown on a Gapolar c-face f₁. However, conditions are different around an outer endof the epitaxial wafer from in an inner portion (i.e., a flat portion inthe c-face growth). Around the outer end of the epitaxial wafer, anitride semiconductor crystal 2 b whose surface is a face f₂ tilted fromthe c-face f₁ is grown. In FIG. 1, there is shown a schematic manner inwhich the nitride semiconductor crystal 2, i.e., the GaN crystal beginsto grow from the seed crystal substrate 1, i.e., the heterogeneoussubstrate (sapphire substrate, for example), and the crystal growthprogresses as indicated by dotted lines, with time. That is, the c-face(growth face) f₁ of the nitride semiconductor crystal 2 a grown over theprincipal surface (c-face, for example) of the seed crystal substrate 1grows gradually as indicated by f₁₋₁, f₁₋₂ and f₁₋₃, and also the face(growth face) f₂ of the nitride semiconductor crystal 2 b around theouter end grown on the face f₂ tilted from the c-face f₁ likewise growsgradually as indicated by f₂₋₂, f₂₋₂ and f₂₋₃.

In general, different crystal faces make their surface dangling bonddensities or surface reconstructions different, therefore making theirimpurity incorporation efficiencies significantly different. For thisreason, in the nitride semiconductor epitaxial wafer growth, the dopedimpurity concentrations in the nitride semiconductor crystal 2 a in theinner portion of the wafer and in the nitride semiconductor crystal 2 baround the outer end of the wafer are significantly different.Contacting the crystals with different impurity concentrations causesstress therebetween due to the difference between the impurityconcentrations affecting elastic and plastic properties, thermalproperties, lattice constants and the like of the crystals. That is, inthe wafer with the growth face therearound different from the c-face,the region of the crystal 2 b with the impurity concentration differentfrom the impurity concentration of the c-face grown crystal 2 a formsaround the outer end, has significant inherent stress in the outer end,and leads to the occurrence of cracking. Due to the size of thatdifferent impurity concentration region around the outer end and thevalue of the impurity concentration in the outer end varying accordingto apparatus configuration and growth conditions, it is difficult todetermine the critical film thickness of the GaN thin film over theheterogeneous substrate.

Also, the use of the nitride semiconductor freestanding substrate itselfas the seed crystal substrate causes crystal growth on a face other thanthe c-face around the outer end of the nitride semiconductor crystal.This causes stress to occur, leading to cracking. That is, as shown inFIG. 2, installing a seed crystal substrate (freestanding nitridesemiconductor substrate) 1 on an installation surface 4 of a tray 3, andgrowing a nitride semiconductor crystal 2 over the seed crystalsubstrate 1, a nitride semiconductor crystal 2 b causing the occurrenceof stress grown on the face f₂ tilted from the c-face f₁ grows around anouter end of a c-face grown nitride semiconductor crystal 2 a grown on aprincipal surface (c-face, for example) of the seed crystal substrate 1.In addition, the use of the nitride semiconductor freestanding substrateas the seed crystal substrate may often cause strain due to the stressinherent to the nitride semiconductor freestanding seed crystalsubstrate itself as well as the stress in the outer end, or cause stressbetween the nitride semiconductor freestanding substrate (the seedcrystal substrate) and the nitride semiconductor layer (the grown layer)grown thereover. These lead to the occurrence of cracking during thegrowing and cooling.

As described above, in either case of the thin film growth of thenitride semiconductor over the heterogeneous seed crystal substrate orthe growth of the nitride semiconductor crystal over the nitridesemiconductor freestanding seed crystal substrate, when deliberatelygrowing the intended nitride semiconductor crystal over the principalsurface or the like of the seed crystal substrate, a nitridesemiconductor crystal other than the deliberately grown nitridesemiconductor crystal exists as, for example, the outer end of thenitride semiconductor crystal or the seed crystal nitride semiconductorfreestanding substrate. These lead to the occurrence of stress, and tothe occurrence of cracking in the nitride semiconductor crystal.

Accordingly, it is an object of the invention to provide a nitridesemiconductor crystal producing method capable of suppressing theoccurrence of cracking in the nitride semiconductor crystal and therebyensuring the enhancement of the nitride semiconductor crystal yield, anda nitride semiconductor epitaxial wafer and a nitride semiconductorfreestanding substrate realized by that method.

According to a first embodiment of the invention, a nitridesemiconductor crystal producing method comprises

growing a nitride semiconductor crystal over a seed crystal substrate,while applying an etching action to an outer end of the seed crystalsubstrate during the growing of the nitride semiconductor crystal.

According to a second embodiment of the invention, in the nitridesemiconductor crystal producing method according to the firstembodiment, the nitride semiconductor crystal producing method furthercomprises

installing the seed crystal substrate in a container with an innersurface and a sidewall for surrounding an outer side of the seed crystalsubstrate,

wherein an environment adjacent to a portion of the inner surface of thecontainer, which is not contacted with the seed crystal substrate atgrowth initiation, is an environment to which an etching action isapplied during the nitride semiconductor crystal growing, so that thenitride semiconductor crystal is grown in no contact with the portion ofthe inner surface of the container throughout an entire period ofcrystal growth, and in a cross-sectional shape similar to an innercross-sectional shape of the container.

According to a third embodiment of the invention, in the nitridesemiconductor crystal producing method according to the secondembodiment, the inner surface of the container, which is not contactedwith the nitride semiconductor crystal, includes a side surface of thesidewall.

According to a fourth embodiment of the invention, in the nitridesemiconductor crystal producing method according to the second or thirdembodiment, the inner surface of the container, which is not contactedwith the nitride semiconductor crystal, includes a surface on a side inwhich the seed crystal substrate is installed.

According to a fifth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to fourth embodiments, the environment adjacent to the portion ofthe inner surface of the container, which is not contacted with thenitride semiconductor crystal, is an environment in which the etchingaction weakens with distance from a side surface of the sidewall.

According to a sixth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to fifth embodiments, the nitride semiconductor crystal is grownin an environment of coexistence of growth and etching, and the etchingaction is strengthened by diluting a growing raw material adjacent tothe portion of the inner surface of the container, which is notcontacted with the nitride semiconductor crystal.

According to a seventh embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of the sixthembodiment, the growing raw material is diluted by feeding an inert gascontaining nitrogen, argon or helium.

According to an eighth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to fifth embodiments, the etching action is applied by feeding anetching gas or liquid adjacent to the portion of the inner surface ofthe container, which is not contacted with the nitride semiconductorcrystal.

According to a ninth embodiment of the invention, in the nitridesemiconductor crystal producing method according to the eighthembodiment, the etching gas contains at least any one of hydrogen,chlorine, and hydrogen chloride.

According to a tenth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to fourth embodiments, the nitride semiconductor crystal is grownby feeding a substance for producing an etching species through acatalytic action, and at least a portion of the inner surface of thecontainer, which is not contacted with the nitride semiconductorcrystal, comprises a catalyst having the catalytic action, to therebydevelop the etching action.

According to an eleventh embodiment of the invention, in the nitridesemiconductor crystal producing method according to the tenthembodiment, the substance for producing the etching species through thecatalytic action is a hydrogen gas.

According to a twelfth embodiment of the invention, in the nitridesemiconductor crystal producing method according to the tenth oreleventh embodiment, the catalyst having the catalytic action is a metalor a metal nitride.

According to a thirteenth embodiment of the invention, in the nitridesemiconductor crystal producing method according to the twelfthembodiment, the metal is any of Ti, Zr, Nb, Ta, Cr, W, Mo, or Ni.

According to a fourteenth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to thirteenth embodiments, a distance between the etchingaction-producing inner surface of the container, which is not contactedwith the nitride semiconductor crystal, and the nitride semiconductorcrystal, ranges from 1 to 10 mm, for a period from crystal growthinitiation until termination.

According to a fifteenth embodiment of the invention, in the nitridesemiconductor crystal producing method according to any one of thesecond to fourteenth embodiments, an angle between a side surface of thesidewall and a placement surface of the container on which the seedcrystal substrate is placed ranges greater than 90 degrees and notgreater than 135 degrees, so that the inner cross section of thecontainer is shaped to expand toward its opening side, and

the nitride semiconductor crystal is grown having a nitrogen face as agrowth face, while expanding its diameter.

According to a sixteenth embodiment of the invention, a nitridesemiconductor epitaxial wafer comprises:

a plate-like seed crystal substrate; and

a nitride semiconductor crystal grown over the seed crystal substrate,

wherein the nitride semiconductor crystal includes a nitridesemiconductor crystal grown in a principal surface direction of the seedcrystal substrate, but around an outer end of the nitride semiconductorcrystal grown in the principal surface direction, there is included nonitride semiconductor crystal grown in a face direction tilted from theprincipal surface, and having a higher impurity concentration than thenitride semiconductor crystal grown in the principal surface direction,or

even when there is included the nitride semiconductor crystal having thehigher impurity concentration around the outer end, a growth thicknessof the nitride semiconductor crystal having the higher impurityconcentration around the outer end is less than one tenth of a growththickness of the nitride semiconductor crystal grown in the principalsurface direction.

According to a seventeenth embodiment of the invention, in the nitridesemiconductor epitaxial wafer according to the sixteenth embodiment, theseed crystal substrate is a sapphire substrate, and the nitridesemiconductor crystal is a GaN layer, and

let a curvature radius of the nitride semiconductor epitaxial wafer be R(m), a thickness of the GaN layer be t (μm), a thickness of the sapphiresubstrate be Y (μm), and a coefficient be A, then the followingrelations (1) and (2) are satisfied.

R=A/t  (1)

A>0.00249×Y ^(1.58483)  (2)

According to an eighteenth embodiment of the invention, a plate-likenitride semiconductor freestanding substrate comprises

a nitride semiconductor crystal grown in a principal surface directionof the nitride semiconductor freestanding substrate,

wherein around an outer end of the nitride semiconductor freestandingsubstrate, there is included no nitride semiconductor crystal having ahigher impurity concentration than the nitride semiconductor crystalgrown in the principal surface direction, or

even when there is included the nitride semiconductor crystal having thehigher impurity concentration around the outer end, a growth thicknessof the nitride semiconductor crystal having the higher impurityconcentration around the outer end is less than one tenth of a growththickness of the nitride semiconductor crystal grown in the principalsurface direction.

(Points of the Invention)

According to the invention, it is possible to suppress the occurrence ofcracking in the nitride semiconductor crystal grown over the seedcrystal substrate and thereby ensure the enhancement of the yield.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, the preferred embodiments according to the invention will beexplained below referring to the drawings, wherein:

FIG. 1 is a schematic cross-sectional view showing a vicinity of anouter end of a nitride semiconductor layer grown over a heterogeneousseed crystal substrate by a conventional method;

FIG. 2 is a schematic cross-sectional view showing a vicinity of anouter end of a nitride semiconductor layer grown over a nitridesemiconductor freestanding seed crystal substrate by a conventionalmethod;

FIGS. 3A and 3B are a cross-sectional view and a plan view,respectively, showing an essential construction of a nitridesemiconductor crystal producing method in one embodiment according tothe invention;

FIG. 4 is a cross-sectional view showing an essential construction of anitride semiconductor crystal producing method in another embodimentaccording to the invention;

FIG. 5 is a cross-sectional view showing an essential construction of anitride semiconductor crystal producing method in another embodimentaccording to the invention;

FIG. 6 is a cross-sectional view showing an essential construction of anitride semiconductor crystal producing method in another embodimentaccording to the invention;

FIG. 7 is a schematic view of a conventional HVPE apparatus used incomparative example 1;

FIGS. 8A to 8C are graphs showing properties of a GaN layer formed overa sapphire substrate in example 1 according to the invention andcomparative example 1, wherein FIG. 8A is a graph showing a relationshipbetween GaN layer thickness and yield; FIG. 8B is a graph showing arelationship between GaN layer thickness and (0002) diffraction fullwidth at half maximum; and FIG. 8C is a graph showing a relationshipbetween GaN layer thickness and (10-12) diffraction full width at halfmaximum;

FIG. 9 is a diagram showing an image of a cross section of a GaN crystalformed over a seed crystal substrate observed by a fluorescencemicroscope in comparative example 1;

FIG. 10 is a schematic view of an HVPE apparatus for implementing thenitride semiconductor crystal producing methods according to theinvention used in example 1;

FIGS. 11A to 11C are graphs showing results of measuring curvature radiiR of epitaxial wafers produced when growing GaN layers with variousthicknesses over sapphire substrates with various thicknesses, using anexample 1 method and a comparative example 1 method, wherein FIG. 11A isa graph showing the results when growing the GaN layers over a sapphiresubstrate with 50 mm diameter and 350 μm thickness; FIG. 11B is a graphshowing the results when growing the GaN layers over a sapphiresubstrate with 100 mm diameter and 900 μm thickness; and FIG. 11C is agraph showing the results when growing the GaN layers over a sapphiresubstrate with 150 mm diameter and 1500 μm thickness;

FIG. 12 is a schematic cross-sectional view showing a vicinity of anouter end of a GaN layer grown over a heterogeneous seed crystalsubstrate using the HVPE apparatus of FIG. 10 in example 1;

FIG. 13 is a process diagram illustrating a VAS method for producing aGaN freestanding substrate used in example 10 and comparative example 2;

FIG. 14 is a graph showing a relationship between thickness and yield ofa GaN crystal grown over a heterogeneous substrate in example 10according to the invention and comparative example 2;

FIG. 15 is a graph showing a relationship between thickness and yield ofa GaN growth layer grown over a GaN substrate in example 11 according tothe invention and comparative example 3;

FIG. 16 is a schematic cross-sectional view illustrating a vicinity ofan outer end of a GaN crystal grown over a GaN freestanding substrate asa seed crystal in example 11 according to the invention;

FIG. 17 is a graph showing a relationship between thickness and yield ofa GaN growth layer grown over a GaN substrate in example 12 according tothe invention; and

FIG. 18 is a cross-sectional view showing a horizontal flow type growingapparatus in modification 9 according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Finding)

The inventors have made an earnest study to solve the drawback thatcracking tends to occur during nitride semiconductor growing. As aresult, it has been found that as described above, when grown over aseed crystal substrate made of a heterogeneous substrate or a nitridesemiconductor freestanding substrate, crystals having propertiesdifferent from those of a crystal to be grown over an intended growthface (e.g. over a principal surface of the seed crystal substrate) arepresent, for example around an outer end of a deliberately grown nitridesemiconductor crystal, or as the seed crystal substrate itself, thosecrystals cause stress to occur between them and the nitridesemiconductor crystal over the intended growth face, and this leads tocracking during the growth.

Based on this finding, the inventors have arrived at a nitridesemiconductor crystal producing method of the invention, which appliesan etching action to a growing portion (in particular, a growing outerend) over a face other than the intended growth face of the nitridesemiconductor crystal to suppress the growth of the outer end whichcauses stress, or gradually etches the growing nitride semiconductorfreestanding seed crystal substrate which causes stress, to therebysuppress cracking during the growth of the nitride semiconductorcrystal.

Below are described a nitride semiconductor crystal producing method, anitride semiconductor epitaxial wafer, and a nitride semiconductorfreestanding substrate in an embodiment according to the invention.

As shown in, for example FIGS. 3A and 3B, the nitride semiconductorcrystal producing method in one embodiment according to the inventioninstalls a seed crystal substrate 1 on a bottom wall 5 b in a crucibleshape or shallow cup-shaped crucible container 5 having a sidewall 5 asurrounding an outer side of the seed crystal substrate 1, feeds a rawmaterial gas G onto a growth surface of the seed crystal substrate 1(onto a principal surface of the seed crystal substrate 1), and grows anitride semiconductor crystal 2 over the seed crystal substrate 1. Anenvironment or atmosphere adjacent to an inner surface portion of thecontainer 5 (in FIGS. 3A and 3B, a side surface 6 of a sidewall 5 a anda portion adjacent to the side surface 6 of a bottom surface 7 of abottom wall 5 b) which is not contacted with the seed crystal substrate1 at the initiation of the growth is defined as an environment oratmosphere to apply an etching action to the growing nitridesemiconductor crystal 2. This allows for the nitride semiconductorcrystal 2 being grown in no contact with the portion of the innersurface of the container 5 throughout an entire period of crystalgrowth, and in a cross-sectional shape similar to an innercross-sectional shape (a circular shape in FIGS. 3A and 3B) of thecontainer 5.

It is preferred that the above described inner surface of the containerwhich is not contacted with the nitride semiconductor crystal includesthe side surface of the sidewall of the container. In addition, if thenitride semiconductor crystal is grown over the seed crystal substratewhich is the nitride semiconductor freestanding substrate, it ispreferred that the above described inner surface of the container whichis not contacted with the nitride semiconductor crystal includes asurface on a side in which the nitride semiconductor freestandingsubstrate is installed, in addition to the side surface of the sidewallof the container.

It is preferred that the above described etching environment or etchingatmosphere adjacent to the portion of the inner surface of thecontainer, which is not contacted with the nitride semiconductorcrystal, is such that the etching action weakens with distance from theinner surface of the container. In the case where the etching actiondoes not weaken with distance from the inner surface, much of thenitride semiconductor crystal grown during a growth period may beetched.

Further, even when the etching action attenuates with distance from theinner surface, if the etching action is too strong even at a distancefrom the inner surface, the layer grown over the principal surface ofthe nitride semiconductor crystal is lost. In addition, if the etchingaction is too weak, growth at an unintended face (end face, for example)occurs in the grown layer of the nitride semiconductor crystal, causingstress in the growing nitride semiconductor crystal, to crack thenitride semiconductor crystal. From these, the strength of the etchingaction and the degree of the weakening (attenuation) of the etchingaction with distance from the inner surface of the container arerequired to be appropriately selected to prevent the occurrence ofexcessive stress, or remove stress.

The presence of this etching action adjacent to the side surface 6 ofthe crucible shaped container 5 as shown in FIGS. 3A and 3B, forexample, suppresses the growth around the outer end (end face) 8 of thenitride semiconductor crystal 2 grown over the seed crystal substrate 1.That is, the growth of the crystal of the outer end 8 different inimpurity concentration from the crystal of the deliberately grown face(for example, c-face) 9 can be suppressed, so that the occurrence ofcracking due to stress caused by the crystal of the outer end 8 can besuppressed. In this case, it is feasible to grow the nitridesemiconductor crystal with the cross-sectional shape substantiallysimilar to the cross-sectional shape (circular shape in FIGS. 3A and 3B)of the inner surface of the container, and in no contact with the innersurface portion of the container throughout the entire period of growth,which is not contacted with the seed crystal substrate at the initiationof the growth. Since the outer portion of the seed crystal substrate 1is not open, and the sidewall 5 a of the container 5 is provided tosurround the outer side of the seed crystal substrate 1, it is possibleto form and maintain the etching environment or etching atmosphere whichallows the selective etching of the nitride semiconductor crystal of theouter end 8.

In this case, in the above etching action, the growth rate of thecrystal of the outer end is preferably 0. This is a state in which theetching action and the growth are balanced. In other words, since theouter end is the etching action applying environment or atmosphere, thegrowth of the outer end can be suppressed. However, if the growth ratein the normal direction to the growth face of the outer end is less thanone-tenth of the growth rate in the normal direction to the intendedgrowth face (e.g., the growth face grown in the normal direction to theprincipal surface (for example, c-face) of the seed crystal substrate,the effect of preventing cracking can be provided. In the case ofone-tenth or more, excessive stress occurs, increasing the probabilityof occurrence of cracking.

Further, when causing the outer end to etch, the etching rate in thenormal direction to the etching face is preferably not more than thegrowth rate of the intended growth face. This is because even if theetching rate of the outer end is greater than the growth rate of theintended growth face, the effect of preventing cracking is provided, butthe size of the finally resulting crystal is very small.

Also, when growing the nitride semiconductor over the nitridesemiconductor free-standing substrate which is the seed crystalsubstrate, the presence of this etching action on the face on the sidein which the nitride semiconductor free-standing substrate is installedallows the gradual etching of the back side of the seed crystalsubstrate during growth. Therefore, in a stage a new nitridesemiconductor growth layer is grown to be thick on the surface side ofthe seed crystal substrate, the stress due to the seed crystal substratecan be removed or reduced, to suppress the occurrence of cracking duringthe growth.

This case has a good result if the etching rate of the back side of theseed crystal substrate is not less than one hundredth of the growth rateon the intended growth face. Also, when the etching rate of the backside of the seed crystal substrate is high, it is assumed that aftergrowth the nitride semiconductor becomes smaller in entire thicknessthan the initial nitride semiconductor freestanding substrate. To avoidsuch a situation, the etching rate of the back side of the seed crystalsubstrate is preferably not more than half the growth rate on theintended growth face.

In addition, the above etching action is applied preferablysymmetrically with respect to a certain point, line or face in thecrystal grown in order to maintain stress balance during the growth.When one side of the crystal substrate, for example, the disc-shapedseed crystal substrate is the intended growth face, it is preferred touse the container which is circular in cross section, so that theetching action is applied evenly to the end face around the crystal.Also, when one side of the seed crystal substrate having such apolygonal plate shape as a square plate shape, hexagonal plate shape,etc. is the principal growth face, it is preferred to use the containerwhose cross-sectional shape is similar to the cross-sectional shape ofthe seed crystal substrate, so that the etching action is applied evenlyto the end face around the nitride semiconductor crystal over the seedcrystal substrate. Further, the seed crystal substrate of the presentinvention is not limited to the plate shape, but seed crystals of eachkind are included. For example, when a circumferential face or polygonalface of the seed crystal substrate having a columnar shape or polygonalpyramid is the principal growth face, it is preferred to apply theetching action evenly to a side surface relative to the principal growthface.

Further, when applying the etching action to the back side of the seedcrystal substrate, it is preferred to apply the etching action evenly toits entire face, or apply the etching action symmetrically with respectto a point or a line in the face. However, in order to, apply theetching action to the back side of the seed crystal substrate that facesthe inner surface of the container, it is necessary to space the backside of the seed crystal substrate a constant distance from the innersurface of the container, for example it is necessary to install theseed crystal substrate in the container via a block smaller than theseed crystal substrate. In this case, it is preferred to use a pluralityof the sufficiently small blocks, so that the area of the face hidden bycontact with the block is not more than one-tenth of the area of theentire back side of the seed crystal substrate. It is because if thisratio is too high, the effect of stress relaxation due to the etching ofthe seed crystal substrate cannot sufficiently be provided.

Specifically, for example, as shown in FIG. 6, the seed crystalsubstrate 1 is installed via a plurality of blocks 17 on a bottom wall10 b of a container 10 having a sidewall 10 a, and a gas g is introducedto apply the etching action to the back side of the seed crystalsubstrate 1. In FIG. 6, a middle portion of the bottom wall 10 b of thecontainer 10 is connected with a feed pipe 18 to feed the gas g, so thatthe gas g introduced from the feed pipe 18 flows between the bottom wall10 b of the container 10 and the seed crystal substrate 1. The back side16 of the seed crystal substrate 1 is etched by the gas g. In addition,the gas g that flows along the back side 16 is injected from between thesidewall 10 a of the container 10 and the outer portion of the seedcrystal substrate 1, to etch mainly the nitride semiconductor crystal 2of the outer end 8, of the nitride semiconductor crystal 2 grown overthe seed crystal substrate 1.

The above etching action is preferably steadily applied, but mayintermittently be applied. For example, one preferred method to achievethe object of the present invention is to cause the gas with the etchingaction to flow steadily around the wafer, but in this case it isnecessary to locate a gas injection hole around the entire wafer. On theother hand, the entire wafer perimeter or the entire wafer back side mayalso be etched intermittently by providing an injection hole for a gashaving the etching action in only a portion of the wafer perimeter, orspraying a gas having the etching action on the wafer perimeter in onedirection, and rotating the wafer. If the etching action is sufficient,this method has the merit that apparatus construction is convenient.

There are several ways to develop the etching action described above.One preferred method is to grow the nitride semiconductor in anenvironment of coexistence of growth and etching, dilute the growing rawmaterial adjacent to the face to which the above etching action isapplied, and thereby strengthen the above etching action. As theenvironment of coexistence of growth and etching, there is, for example,a case of vapor phase growth such as MOVPE growth, HVPE growth or thelike, in which hydrogen, chlorine, hydrogen chloride or the like isadded to a growth atmosphere. In this case, the growing raw material isdiluted preferably by feeding an inert gas, such as nitrogen, argon,helium or the like. The growing raw material in the raw material gas Gis diluted by, for example, in FIGS. 3A and 3B, adding in the rawmaterial gas G a gas having the etching action, such as hydrogen,chlorine, hydrogen chloride or the like in addition to the growing rawmaterial (group III raw material and group V raw material), and feedingthe inert gas, such as nitrogen, argon, helium or the like to thevicinity of the side surface 6 of the container 5.

A desirable value of an added amount of these diluting gases (inertgases) to provide the appropriate etching action is preferably set torealize the above growth rate and the above etching rate in the range offrom one-tenth to ten times the feed amount of the group III rawmaterial, though varying according to growth conditions.

Further, the present invention may also be applied to growth using asolution performed in a closed system, as in a Na flux method, anammonothermal synthesis method or the like. In these cases, the etchingaction is developed by forcedly introducing the solution which is lowerin raw material solubility than the solution in the vicinity of thedeliberately grown face, into the vicinity of the face to which theetching action is applied.

The above described etching action may be developed by feeding a gas orliquid (solution) with the etching action into the vicinity of the faceto which the etching action is applied.

The above described gas with the etching action preferably contains atleast any one of hydrogen, chlorine, or hydrogen chloride. In order toprovide the appropriate etching action, a desirable value of an addedamount of these etching gases is preferably set to realize the abovegrowth rate and the above etching rate in the range of from one-tenth toten times the feed amount of the group III raw material, though varyingaccording to growth conditions.

In the Na flux method or ammonothermal synthesis method, the solution,which is lower in growing raw material solubility than the solution inthe vicinity of the deliberately grown face, is liquid (solution) havingthe etching action in the same manner as described above.

In FIG. 4, there is shown one embodiment of the above method which feedsthe gas with the etching action in the vicinity of the face to which theetching action is applied. A container in this embodiment is composedmainly of a cup-shaped container 10 having a sidewall 10 a, and a tray 3to place a seed crystal substrate 1. A placement surface (installationsurface) 4 of the tray 3 and the side surface of the sidewall 10 a areconfigured as an inner surface of the shallow cup-shaped container, sothat the gas g with the etching action is introduced from a bottomsurface perimeter of the inner surface of this container to the outerportion in this container.

A middle portion of a bottom wall 10 b of the container 10 is connectedwith a feed pipe 11 to feed the gas g with the etching action. The feedpipe 11 is inserted therein and a support shaft 13 to support the tray 3is provided therein. The tray 3 on the support shaft 13 is arranged witha predetermined gap between it and the bottom wall 10 b of the container10. The gas g fed to the middle portion of the bottom wall 10 b of thecontainer 10 from the feed pipe 11 flows radially around the supportshaft 13 and between the tray 3 and the bottom wall 10 b, and flows outfrom an annular gas injection hole (gas outlet) 19 formed between theouter surface of the tray 3 and the inner surface of the sidewall 10 a.On the other hand, the raw material gas G containing the growing rawmaterial is fed to the growth face (the principal surface) of the seedcrystal substrate 1 installed on the placement surface 4 of the tray 3,to grow the nitride semiconductor crystal 2 over the seed crystalsubstrate 1. The outer end 8 of the nitride semiconductor crystal 2 issubjected to a predetermined etching due to the gas g with the etchingaction injected from the gas injection hole 19.

Also, the etching action may be developed by growing the nitridesemiconductor crystal while feeding a substance for producing an etchingspecies through a catalytic action, and by making at least a portion ofthe inner surface of the container, which is not contacted with theabove described nitride semiconductor crystal, from a catalyst. Becauseof requiring no local gas or liquid introduction, this method has themerit of being able to be implemented with a more convenient apparatusthan the previously mentioned two methods.

The above described substance for producing the etching species throughthe catalytic action is preferably a hydrogen gas, and the catalyst ispreferably a metal or a metal nitride.

The hydrogen gas etching is assumed to be performed by the hydrogen gascontacting the catalyst made of the metal or metal nitride in a hightemperature condition to produce atomic hydrogen with the strong etchingaction, and the resulting atomic hydrogen diffusing and reaching thenitride semiconductor crystal. The atomic hydrogen is unstable, reactsin a short time and disappears. Therefore, the etching action due to theatomic hydrogen attenuates rapidly with increasing distance from theinner surface of the container, to etch only the nitride semiconductorcrystal lying in the inner side at a constant distance from the innersurface of the container. That is, the vicinity of the inner surface ofthe container is the atmosphere in which the etching action weakens withdistance from the inner surface of the container, therefore allowingappropriate etching without much of the nitride semiconductor crystalbeing etched due to excessive etching of the nitride semiconductorcrystal, or without the occurrence of cracking in the nitridesemiconductor crystal due to insufficient etching.

Further, as the above described catalyst metal, Ti (titanium), Zr(zirconium), Nb (niobium), Ta (tantalum), Cr (chromium), W (tungsten),Mo (molybdenum), and Ni (nickel) are preferred. For example, it is onepreferred embodiment of the present invention to nitride the innersurface of the container made of the above described metal solidmaterial in the nitride semiconductor crystal growing apparatus prior togrowing the nitride semiconductor, and then perform the crystal growth.In addition, the inner surface of the container, which is not contactedwith the nitride semiconductor crystal, may be formed with a film of theabove described metal or metal nitride.

In this method, for example, in FIGS. 3A and 3B, the hydrogen gas may beadded as the raw material gas G, and the side surface 6 of the container5 may be formed of the above described metal or metal nitride. Also, forexample, in FIG. 4 or 6, the hydrogen gas may be contained in the rawmaterial gas G, and the inner surface of the sidewall 10 a of thecontainer 10, or the bottom wall 10 b of the container 10 in FIG. 4 or 6may be formed of the above described metal or metal nitride.

The distance between the sidewall of the container that is not contactedwith the above-mentioned nitride semiconductor crystal and the nitridesemiconductor crystal is preferably in the range of 1 to 10 mm for theperiod from the initiation of growth until the end of the growth. Ifthis distance is shorter than 1 mm, a slight variation in growthconditions causes the nitride semiconductor crystal to be contacted withthe inner surface of the container, to fix the container and the nitridesemiconductor crystal, and due to the fixation, stress occurs to crackthe nitride semiconductor crystal. On the other hand, if this distanceis longer than 10 mm, the steep attenuation of the etching action islost, and only the outer end growth cannot selectively be inhibited byetching. The distance between the inner surface of the container and thenitride semiconductor crystal is a distance d as shown in FIG. 3B, forexample.

In the nitride semiconductor epitaxial wafer with the nitridesemiconductor crystal grown over the plate shaped seed crystal substraterealized by the above described nitride semiconductor crystal producingmethod, the nitride semiconductor crystal does not include a nitridesemiconductor crystal grown in a principal surface direction of the seedcrystal substrate, and around an outer end of the nitride semiconductorcrystal grown in the principal surface direction, a nitridesemiconductor crystal grown in a face direction tilted from theprincipal surface, and having a higher impurity concentration than thenitride semiconductor crystal grown in the principal surface direction.Or, even when the nitride semiconductor crystal includes them, thenitride semiconductor epitaxial wafer is produced such that a growththickness of the nitride semiconductor crystal having the higherimpurity concentration is less than one tenth of a growth thickness ofthe nitride semiconductor crystal grown in the principal surfacedirection.

For this reason, during the growth and during cooling, the occurrence ofcracking of the nitride semiconductor crystal is suppresseddramatically, and a high yield is feasible. The term “high impurityconcentration” herein means that although the nitride semiconductorcrystal grown in the principal surface direction which is the crystalover the intended face has its in-plane impurity concentrationdistribution to some degree, the impurity concentration is exceedinglyhigher (for example, 2 times or more) than a maximum value of theimpurity concentration in that distribution, but does not mean animpurity concentration value exceedingly higher than a maximum value ofthe impurity concentration within the range of the impurityconcentration distribution in the crystal over the intended principalsurface or in the crystal over the principal surface. Even when growingthe nitride semiconductor crystal with a high impurity concentrationaround the outer end of the nitride semiconductor epitaxial wafer, ifits growth thickness is less than one-tenth of the growth thickness ofthe nitride semiconductor crystal grown in the principal surfacedirection, the occurrence of cracking during the growth and duringcooling is suppressed, and further the warpage of the nitridesemiconductor epitaxial wafer can also be reduced.

In the above-described nitride semiconductor epitaxial wafer, inparticular, in the epitaxial wafer with the GaN layer grown over thesapphire substrate, the epitaxial wafer with warpage smaller than theconventional epitaxial wafer can be realized. Typically, when the GaNlayer is grown over the sapphire substrate, when the surface of the GaNlayer is faced upward, the epitaxial wafer is warped upward in a convexshape. Around the outer end of the nitride semiconductor epitaxial waferfabricated by the conventional producing method, the nitridesemiconductor crystal with high impurity concentration is grownone-tenth or more of the growth thickness of the nitride semiconductorcrystal grown in the principal surface direction, and the radius ofcurvature of the nitride semiconductor epitaxial wafer is small, and theamount of warpage is increased. On the other hand, the nitridesemiconductor epitaxial wafer produced by the producing method in theembodiment according to the present invention has a large radius ofcurvature, and the amount of warpage is reduced.

In FIGS. 11A to 11C, there are shown results of measurement of theradius R of curvature of epitaxial wafers produced when growing GaNlayers with various thicknesses t over sapphire substrates with variousthicknesses Y, using the growth method of the present invention and theconventional producing method. FIG. 11A shows data when the GaN layersare grown over the sapphire substrate of 50 mm diameter and 350 μmthickness, FIG. 11B shows data when the GaN layers are grown over thesapphire substrate of 100 mm diameter and 900 μm thickness, and FIG. 11Cshows data when the GaN layers are grown over the sapphire substrate of150 mm diameter and 1500 μm thickness.

The relationship between the growth thickness of the nitridesemiconductor crystal grown in the principal surface direction of thenitride semiconductor epitaxial wafer and the warpage of the nitridesemiconductor epitaxial wafer is studied. It is found that the followingrelation (1) holds. Let a curvature radius of the epitaxial wafer be R(m), a thickness of the GaN layer be t (μm), and a coefficient be A,then the curvature radius R of the epitaxial wafer can be written asfollows.

R=A/t  (1)

From these results, by comparison of the epitaxial wafer produced by theproduction method of the present invention and the epitaxial waferproduced by the conventional producing method, it is clear that thesapphire substrate thickness Y (μm) and the coefficient A has acorrelation expressed by the following formula.

In the case of the GaN layer over the sapphire grown by the conventionalmethod, when the thickness of the sapphire substrate is Y (μm), thecoefficient A is

A≦0.00249×Y ^(1.58483)  (3)

but by using the nitride semiconductor crystal producing method in theembodiment according to the invention,

A>0.00249×Y ^(1.58483)  (2)

Thus, it is clear that the curvature radius can be increased more.

The epitaxial wafer with the large curvature radius, i.e. the smallamount of warpage is advantageous in the case of forming a lightemitting diode or a transistor structure on the above described GaNlayer, and applying a photolithographic process to this. This is becausein the photolithography process, if the epitaxial wafer is warpedsignificantly, there are such adverse effects that the resolution of anelement pattern transferred to the epitaxial wafer is degraded, and itis impossible to form a fine element, and also the yield in thephotolithography process decreases.

Also the nitride semiconductor freestanding substrate realized by theabove described nitride semiconductor crystal producing method does notinclude around an outer end thereof a nitride semiconductor crystalhaving a higher impurity concentration than the nitride semiconductorcrystal grown in the principal surface direction of the nitridesemiconductor freestanding substrate. Or, even when including it, thenitride semiconductor freestanding substrate is produced such that agrowth thickness of the nitride semiconductor crystal having the higherimpurity concentration around the outer end is less than one tenth of agrowth thickness of the nitride semiconductor crystal grown in theprincipal surface direction.

For this reason, during the growth and during cooling, the occurrence ofcracking of the nitride semiconductor crystal is suppresseddramatically, and a high yield is feasible. The meaning of the “highimpurity concentration” herein is the same as in the case of the abovedescribed nitride semiconductor epitaxial wafer.

Although in the above, the GaN crystal whose deliberately grown face isthe Ga polar c-face has been described, the present invention can, inprinciple, be applied to the case that a group DI polar c-face of AlN,InN, BN and a mixed crystal of these, or all other faces thereof are theintended growth face. For example, a face tilted in a direction of ana-axis or m-axis or in a direction between them in the range of 0.1 to 2degrees from the group III polar c-face, or an N polar c-face, a-face,m-face, r-face, or other semipolar faces, or a vicinal face to those Npolar faces or semipolar faces can be the intended growth face.

Further, the present invention can be applied to the case that thenitride semiconductor crystal is grown in the form that the N polarc-face is the surface. In this case, for the seed crystal substrate, theN polar face is arranged as the surface (the deliberately grown face),and crystal growth is performed thereover. In this case, the end face ofthe nitride semiconductor crystal has an opposite slope to that of FIG.2, and the N polar c-face is expanded with the growth. For this reason,the growth of the nitride semiconductor crystal with the N polar facegrowth is a very effective method in order to realize the nitride largediameter semiconductor substrate (nitride semiconductor freestandingsubstrate).

However, in the case of using the conventional art HVPE apparatus, inthe N polar face growth, stress occurs due to the growth around the endface as in the growth over the group III polar face, making it difficultto achieve a high yield.

However, using the nitride semiconductor crystal producing method of thepresent invention allows the growth of the nitride semiconductorfreestanding substrate with a high yield even in the N polar facegrowth.

Specifically, as shown in FIG. 5, an angle θ between a side surface 14of a sidewall 12 a of a container 12 and a placement surface 15 of abottom wall 12 b of the container 12 to place the seed crystal substrate1 ranges greater than 90 degrees and not greater than 135 degrees, andthe inner cross-section of the container 12 is shaped to expand towardits opening, and the nitride semiconductor crystal 2 may be grown withits nitrogen face being the growth face and with its diameter beingexpanded.

In particular, in the N polar face growth, the sidewall is located sothat the angle between the installation surface and the side surface ofthe container is open upward in the range of greater than 90 degrees(the side surface perpendicular to the installation surface) and notgreater than 135 degrees, so that it is possible to form the nitridesemiconductor crystal with the area of the N face being greater thanthat of the seed crystal substrate, while applying the etching action tothe end of the nitride semiconductor crystal. In the case of thisarrangement, since most of the crystal faces tending to appear aroundthe end of the nitride semiconductor crystal have an angle of notgreater than 135 degrees, if the angle of sidewall is greater than 135degrees, the distance between the sidewall and the outer end of thenitride semiconductor crystal increases with the growth of the nitridesemiconductor crystal. With the growth progress, the etching actionweakens and the growth of the nitride semiconductor crystal of the outerend is significant, and cracking tends to occur due to the stress in theouter end. A similar result to that of the conventional method is onlyprovided.

When this angle θ is not greater than 135 degrees, since the distancebetween the sidewall 12 a and the outer end of the nitride semiconductorcrystal 2 tends to be held constant to balance the growth and theetching to the outer end, the growth rate of the outer end is held at alow value, to suppress the occurrence of cracking. In particular, whenthe open angle of the sidewall 12 a is not greater than 120 degrees,since the stable crystal faces of the nitride semiconductor having anangle smaller than this are lessened, the growth rate around the outerend is reduced to substantially zero, and an even higher growth yieldcan be achieved.

As described above, by applying the nitride semiconductor crystalproducing method of the above-described embodiment to the growth of thenitride semiconductor crystal, in the thin film growth over aheterogeneous substrate, it is possible to grow the nitridesemiconductor layer thicker than the conventional without cracking, andit is possible to enhance the yield more significantly than theconventional method. Further, by applying the nitride semiconductorcrystal producing method of the above embodiments to the production ofthe nitride semiconductor freestanding substrate, it is possible tosignificantly reduce failure due to cracking of the nitridesemiconductor crystal. In addition, because the residual strain islessened, the nitride semiconductor epitaxial wafer and the nitridesemiconductor freestanding substrate of the above embodiments aresuitable for the production of light emitting diodes and laser diodes,and fabrication of high electron mobility transistors and hetero bipolartransistors.

In addition, the nitride semiconductor crystal may be grown byappropriately combining the configurations of each container, etc. ofFIGS. 3A to 6 used in the above embodiments. For example, the bottomwall 12 b of the container 12 shown in FIG. 5 may be provided with afeed pipe to feed the gas with the etching action as shown in FIG. 4,and a tray may be provided on the support shaft with the feed pipeinserted therearound, so that the seed crystal substrate 1 shown in FIG.5 may be installed on this tray, and the nitride semiconductor crystal 2expanded in the growth direction as shown in FIG. 5 may be grown.

EXAMPLES

The present invention will be described in more detail by way of thefollowing examples (including modifications), but is not limited tothese examples.

Example 1 and Comparative Example 1 Comparative Example 1

In comparative example 1, using a vertical arrangement HVPE apparatusconfigured similarly to the conventional shown in FIG. 7, over the seedcrystal substrate 1 which is a sapphire substrate is grown a 2 to 20 μmGaN layer via a low-temperature grown GaN buffer layer. The HVPEapparatus is divided into an upper raw material section 32 and a lowergrowth section 33. An outer portion of the raw material section 32 of areactor (growth furnace) 20 for performing crystal growth is providedwith a raw material section peak 30 and an outer portion of the growthsection 33 of the reactor 20 is provided with a growth section peak 31.By the raw material section peak 30, the raw material section 32 in thereactor 20 is heated to roughly 800 degrees Celsius and also by thegrowth section peak 31 the growth section 33 in the reactor 20 is heatedto 500 to 1200 degrees Celsius.

For gas feed from the raw material section 32 toward the growth section33, three system gas feed lines of a group V line (group V gas feedpipe) 23, a group III line (group DI gas feed pipe) 25, anetching/doping line (etching gas/doping gas feed pipe) 24 are installed.From the group V line 23, a NH₃ (ammonia gas) which is a nitrogensource, and a hydrogen gas, nitrogen gas, or a mixture of these gases asa carrier gas are fed. From the group III line 25, HCl and a hydrogengas, nitrogen gas, or a mixture of these gases as a carrier gas are fed.A Ga melt tank 26 for storing metal gallium 27 is located in the middleof the group DI line 25, where the HCl gas and the metal gallium reactto produce a GaCl gas, and the Gad gas is fed to the growth section 33.From the etching/doping line 24, when ungrown and during undoped GaNlayer growth, a mixture of hydrogen and nitrogen gases is introduced,and during n-type GaN layer growth, dichlorosilane (SiH₂Cl₂, 100 ppmconcentration due to hydrogen dilution) which is a Si source and a HClgas and a hydrogen gas and a nitrogen gas are introduced. In addition,from the etching/doping line 24, during baking at a temperature of about1100 degrees Celsius performed in order to remove GaN adhering in thereactor 20 after growth, a hydrogen chloride gas and hydrogen, nitrogenare introduced.

In the growth section 33 in the reactor 20, a tray 3 to rotate at arotational speed of about 3 to 100 rpm is mounted horizontally, and aseed crystal substrate 1 is installed on the installation surface(placement surface) 4 of the tray 3 that faces the outlets of the gasfeed lines 23 to 25. The tray 3 is provided on the rotating shaft(support shaft) 13 arranged vertically, and the tray 3 is rotated by therotation of the rotary shaft 13. After being used for the growth of GaNon the seed crystal substrate 1, the raw material gas is vented to theoutside from the most downstream of the reactor 20. Growth in thereactor 20 is all performed at atmospheric pressure (1 atm) incomparative example 1.

Each line pipe 23, 24, 25, the Ga melt tank 26, and the rotary shaft 13of the tray 3 are made of a high purity quartz, and the tray 3 is madeof a SiC coated carbon. As the sapphire substrate, there is used onehaving the face tilted at 0.3 degrees in the m axis direction from thec-face, and having a thickness of 900 μm, and a diameter of 100 mm.

The HVPE growth is performed as follows: After setting the sapphiresubstrate 1 on the tray 3, pure nitrogen is caused to flow to expel theair in the reactor 20. Next, in the gas in the mixture of 3 slm hydrogengas and 7 slm nitrogen gas, the substrate temperature in the growthsection 33 is held at 1100 degrees Celsius for 10 minutes. Then, at 550degree Celsius substrate temperature, 20 nm low temperature grown GaNbuffer layer is grown at a growth rate of 1200 nm/h. As gases caused toflow at this time, 1 sccm HCl, 2 slm hydrogen, and 1 slm nitrogen arefed from the group III line 25, 1 slm ammonia and 2 slm hydrogen are fedfrom the group V line 23, and 3 slm hydrogen is fed from theetching/doping line 24.

After the growth of the low temperature grown GaN buffer layer, thesubstrate temperature is increased to 1050 degrees Celsius, and a 2 to20 mm undoped GaN layer is grown at a growth rate of 120 μm/h. As gasescaused to flow at this time, 100 sccm HCl, 2 slm hydrogen, and 1 slmnitrogen are fed from the group III line 25, 2 slm ammonia and 1 slmhydrogen are fed from the group V line 23, and 3 slm hydrogen is fedfrom the etching/doping line 24.

After the growth, with 2 slm ammonia and 8 slm nitrogen being caused toflow, the substrate temperature is cooled to around room temperature.Thereafter, the purge nitrogen is performed for several tens of minutes,to produce a nitrogen atmosphere in the reactor 20, and the substrate istaken out.

A plurality of epitaxial wafers with different GaN layer thicknesses inthe range of 2 to 20 μm are made as described above. For each GaN layerthickness epitaxial wafer, twenty wafers are made, and in FIGS. 8A, 8Band 8C, the yield, the average value of the full width at half maximumin (0002) diffraction and the average value of the full width at halfmaximum in (10-12) diffraction by X-ray diffraction (XRD) measurement ofthe GaN layer are indicated by a circle o. Here, the yield is calculatedby deeming the wafer poor when even one crack with a length of 5 mm ormore occurs in the GaN layer.

As shown in FIGS. 8A, 8B and 8C, until the thickness of the GaN layer is4 μm, the yield is almost 100%, and the XRD full width at half maximumdecreases with increasing thickness of the GaN layer. However, when thethickness of the GaN layer is 5 μm or more, cracking begins to occur,and the yield decreases (FIG. 8A). In addition, the occurrence ofcracking causes degradation of the crystallinity of the GaN layer, andthe XRD full width at half maximum increases (FIGS. 8B and 8C).

The cross-section of the GaN layer grown by the above conventional HVPEapparatus is observed by a fluorescence microscope (the light in thevisible light region is observed by applying ultraviolet light). Regionswith different colors of the GaN crystal are seen as shown in FIG. 9. Atthe bottom of FIG. 9, there are shown contour lines of the crystalregions in the image observed by the fluorescence microscopy at the topof FIG. 9. In addition, the image observed by the fluorescencemicroscope shown in FIG. 9 shows the observed image when growing thethick GaN crystal over the GaN freestanding substrate. The thickness ofthe GaN crystal 2 b at the outer end grown over a face tilted from thec-face at a position of 300 μm from the principal surface (c-face) ofthe GaN crystal 2 a is 44 μm, and the thickness of the GaN crystal 2 bis not less than one-tenth the thickness of the GaN crystal 2 a.Significant stress occurs in the outer end of the GaN crystal, and theoccurrence of cracking is observed.

No emission itself corresponding to the band gap of GaN can be observedby the fluorescence microscopy due to ultraviolet region, but the colordifferences are observed due to a change in concentration at a defectlevel resulting from a difference between impurity concentrations. Thatis, as described with reference to FIG. 1 and FIG. 2, the two differentcolor regions in FIG. 9 reflect the difference between impurityconcentrations. Due to each being grown over the separate crystal face,the difference between impurity concentrations results from a differencebetween impurity incorporation efficiencies.

Specifically, in the figure, the light gray part is the GaN crystal 2 agrown on the c-face f₁, while the Oxford gray part is the region of theGaN crystal 2 b grown on the face f₂ tilted from the c-face f₁. Theimpurity concentration in each of these regions is investigated bymicro-Raman measurement. In the GaN crystal 2 a grown on the c-face f₁,it is of n type and is about 0.5×10¹⁸/cm³ to 5×10¹⁸/cm³, while in theGaN crystal 2 b grown on the face f₂ tilted from the c-face f₁, it is ofthe same n type but the impurity concentration is as very high as1×10¹⁹/cm³ to 5×10¹⁹/cm³ which is two times or more that of the GaNcrystal 2 a. Although no doping gas is caused to flow during growth, itis considered that the impurity injected from the constituent member ofthe growth apparatus is incorporated into the crystal during the growth.As a result of SIMS measurement for each of these regions, it is foundthat the n-type conductivity of these is due to the incorporation of Siand oxygen. From this fact, it is assumed that, due to the very highimpurity concentration of the crystal 2 b grown over the face f₂ tiltedfrom the c-face of the outer portion resulting from its higher impurityincorporation efficiency than that of the crystal 2 a on the c-face f₁,stress occurs between the crystal 2 b and the crystal 2 a grown on theflat portion, and this increases film thickness, causes cracking, andleads to yield lowering.

Example 1

In order to suppress the growth of the GaN crystal 2 b with the highimpurity concentration grown around the outer end of the GaN crystal incomparative example 1, a structure portion including the tray 3 and therotary shaft (support shaft) 13 for supporting and rotating the seedcrystal substrate 1 in the HVPE apparatus of FIG. 7 above is alteredinto the structure similar to FIG. 4, resulting in an HVPE apparatus ofFIG. 10 for implementing the method of example 1. That is, the HVPEapparatus shown in FIG. 10 is structured to be able to introduce thepurge gas g into the entire outer portion of the installation surface(placement surface) 4 of the seed crystal substrate 1. In the HVPEapparatus of FIG. 10, the rotary shaft 13 and the feed pipe 11 to feedthe purge gas g are rotated integrally.

Above the annular gas injection hole 19 for the purge gas g formedbetween the outer surface of the tray 3 and the inner surface ofsidewall 10 a is provided the sidewall 10 a up to a height of 3 mm fromthe installation surface 4. The installation surface 4 of the tray 3 andthe side surface of the sidewall 10 a are configured as an inner surfaceof the crucible shape or shallow cup-shaped container. The distancebetween the side surface of the sidewall 10 a and the outer end face ofthe seed crystal substrate 1 is 5 mm.

Using the HVPE apparatus of this example 1, a GaN layer is grown overthe sapphire seed crystal substrate 1 in the same conditions ascomparative example 1 above. The flow rate of each line during lowtemperature grown GaN buffer layer and undoped GaN layer growth at 1050degrees Celsius is the same as in the above-mentioned comparativeexample 1. However, the introduction of 3 slm nitrogen as the purge gasg around the seed crystal substrate 1 is different from in theabove-mentioned comparative example 1.

When producing twenty wafers for each GaN layer thickness of epitaxialwafers having GaN layers of various thicknesses grown in this manner, inFIGS. 8A, 8B and 8C, the yield, the average value of the full width athalf maximum in (0002) diffraction and the average value of the fullwidth at half maximum in (10-12) diffraction by X-ray diffraction (XRD)measurement of the GaN layer are indicated by an x-mark x. Incomparative example 1 using the HVPE apparatus of FIG. 7 to which theconventional method is applied, when the GaN layer thickness exceeds 5μm the yield decreases rapidly, whereas in the GaN layer grown by theHVPE apparatus of FIG. 10 of example 1, until the GaN layer thickness is8 μm, the yield is almost 100%. In example 1, when the thickness of theGaN layer exceeds 8 μm the yield decreases gradually, but the degree ofthe decrease is much more gradual than in comparative example 1, and theyield of 15% is still provided at a thickness of 20 μm. Further, incomparative example 1, at thicknesses of the GaN layer of 5 to 6 μm atwhich the yield decreases, the resulting minimum XRD full width at halfmaximum is 120 seconds in the (0002) diffraction, and is 350 seconds inthe (10-12) diffraction. At thicknesses exceeding this, the XRD fullwidth at half maximum increases. In contrast, in the GaN layer grown bythe HVPE apparatus of example 1, until a thickness of 15 μm, the fullwidth at half diffraction continues to decrease, and the resultingminimum full width at half maximum is 60 seconds in the (0002)diffraction, and is 150 seconds in the (10-12) diffraction.

That is, it is shown that, by using the HVPE apparatus shown in FIG. 10of example 1, the GaN layer thicker than the conventional can be grownat the good yield, and the thick GaN layer grown in this manner has theimproved crystallinity over the conventional.

In FIG. 12 there is schematically shown a result of fluorescencemicroscope observation of a cross-section of the GaN layer grown by theHVPE apparatus of example 1. Although as in the case of FIG. 9 ofcomparative example 1, some differences in color between the GaN crystal2 a on the c-face f₁ and the GaN crystal 2 b on the face f₂ tilted fromthe c-face f₁ can be seen, even in that case the thickness of the GaNcrystal 2 b grown in the normal direction d₂ to the face f₂ tilted fromthe c-face is very thin, and even the maximum thickness thereof is lessthan one-tenth of the GaN crystal 2 a grown in the normal direction d₁to the c-face f₁. That is, as a result of the purge nitrogen of theperimeter of the seed crystal substrate 1, the growth raw material nearthe outer portion of the seed crystal substrate 1 is diluted, while theetching action of the hydrogen gas or the HCl gas is strengthened, sothat the growth rate of the GaN crystal 2 b on the face tilted from thec-face lying around the outer end of the GaN crystal 2 is less thanone-tenth of the growth rate of the GaN crystal 2 a on the c-face. As aresult of the micro-Raman measurement of the GaN crystal 2 in example 1,the impurity concentration of each of the crystal 2 a and the crystal 2b is similar to the impurity concentration in comparative example 1.

From the above results, it is considered that, using the HVPE apparatusof FIG. 10 of example 1 allows suppressing the growth of the GaN crystal2 b with the high impurity concentration on the face tilted from thec-face of the outer portion of the GaN crystal 2, thereby suppressingcracking. Also, as a result, since the thick GaN layer 2 can be grownwhile preventing cracking, the improvement of the crystallinity over theconventional is achieved.

Furthermore, it is found that using the method of example 1 allowsmaking the curvature radius of the epitaxial wafer greater thancomparative example 1, when growing the GaN layer over the sapphireheterogeneous substrate, and after the growth, cooling the epitaxialwafer down to room temperature.

When growing the GaN layer over the sapphire substrate, and when facingthe GaN surface up, the epitaxial wafer is warped upward in a convexshape. For example, when growing the 8 μm GaN layer over the 2 inchdiameter and 350 μm thick sapphire substrate by comparative example 1,its warpage (the difference in height between the center and the end ofthe surface of GaN) is about 120 μm, and the curvature radius of thewafer is then approximately 2.6 m. On the other hand, when using themethod of example 1, when growing the 8 pin GaN layer over the samesapphire substrate, the warpage is as small as 50 μm, and the curvatureradius is as great as 6 m.

Growing GaN layers of varying thicknesses over sapphire substrates ofvarious thicknesses, comparing and examining them, let the thickness ofthe GaN layers be t (μm), it is then clear that the curvature radius R(m) of the GaN layers over the sapphire substrates can, using acoefficient A, be written as follows:

R=A/t  (1)

That is, the epitaxial wafer produced by the production method of thepresent invention satisfies the previously described formula (2), andhas the large radius of curvature, and the small warpage. The epitaxialwafer with the large curvature radius and the small amount of warpage isadvantageous in the case of forming a light emitting diode or atransistor structure on the above described GaN layer, and applying aphotolithographic process to this. This is because in thephotolithography process, if the epitaxial wafer is warpedsignificantly, there are such adverse effects that the resolution of anelement pattern transferred to the epitaxial wafer is degraded, and itis impossible to form a fine element, and also the yield in thephotolithography process decreases.

Example 2

In example 2, in the method of example 1, the flow rate of the abovedescribed nitrogen gas which is the purge gas g to the outer portion ofthe sapphire seed crystal substrate 1 is variously altered within arange of from 2.0 slm to 10 slm, and an experiment similar to that ofexample 1 is performed.

When the purge nitrogen gas flow rate is 2 slm or greater, the resultsfor the yield of the epitaxial wafer, the crystallinity of GaN, and thecurvature radius of the epitaxial wafer are almost the same as theresults in example 1. In addition, at the purge nitrogen gas flow ratesof between 2 to 5 slm, the growth rate of the face tilted from thec-face is in the range of from less than one-tenth of the growth rate onthe c-face to 0. For this reason, the occurrence of stress in the outerportion as in the conventional example is suppressed, and the occurrenceof cracking is suppressed.

Also, at purge nitrogen flow rates of 6 slm or more, substantially thesame results as in example 1 are provided. In this case, however, also,no growth over the face tilted from the c-face in the fluorescencemicroscopy observation of cross section is observed at all. In thiscase, it is determined that the size of the growth region on thesapphire substrate is smaller than in the case of 5 slm purge nitrogengas flow rate, and that rather than growth, etching occurs in the endface of the GaN growth layer. The etching rate is in the range of fromthe growth rate equal to the growth rate on the c-face to one-tenthgrowth rate.

Even if this etching rate is even faster, there is no problem from thepoint of view of yield and crystallinity. However, because if theetching rate is too fast, the size of the finally resulting crystalbecomes very small, it is considered desirable that the etching rate ispractically not more than the growth rate on the intended growth face(c-face).

Here, consider the meaning that when increasing the purge nitrogen gasflow rate, the growth rate on the face tilted from the c-face is firstreduced, and further the etching occurs. In the growth of GaN in thisexample, the growth atmosphere contains hydrogen. It is known that GaNis etched by hydrogen at high temperatures, and it is considered thatthe growth of GaN results from the growth rate exceeding the etchingrate. That is, in the conditions of this example, the growth of GaN isperformed in the environment of coexistence of etching and growth, andby purging the epitaxial wafer perimeter with nitrogen and diluting theraw material gas, the etching action in the perimeter of the GaN crystalis strengthened, and a decrease in the growth rate and the etching isobserved.

Since the above etching acts only on the GaN crystal of the outer end ofthe epitaxial wafer, t is considered that in the range of the flow rateof the purge nitrogen gas in this example, the etching action weakensrapidly with increasing distance from the sidewall inner surface of thecrucible shaped container.

Reference Example

If, in the producing method of Example 2 the flow rate of the purgenitrogen gas is 1 slm or less, the results for the yield of theepitaxial wafer, the crystallinity of GaN, and the radius of curvatureof the epitaxial wafer are substantially the same as the results inComparative Example 1.

This is considered because, due to little flow of the purge gas g, thegrowth rate in the normal direction to the face tilted from the c-faceof the epitaxial wafer is ⅕ or more of the growth rate on the c-face,and the crystal portion with the high impurity concentration grown inthe normal direction to the face tilted from the c-face is 1/10 orgreater of the growth thickness of the crystal on the c-face, andtherefore stress occurs in the outer portion as in the conventionalexample. Thus, it is found that the flow rate of the purge gas g may bemore than 1 slm, preferably 2 slm or more.

Example 3

In example 3, an experiment similar to example 2 is performed bychanging the purge gas to the epitaxial wafer perimeter to argon andhelium, almost the same result as example 2 are provided. From thisresult, the advantageous effect of the present invention is consideredto be provided by using an inert gas other than nitrogen, argon, andhelium as the purge gas.

Example 4

In example 4, the same experiment as in example 2 is performed by usinggases to etch GaN, such as hydrogen, chlorine, and hydrogen chloride asthe purge gas to the epitaxial wafer perimeter. As a result, althoughthe range of the purge gas flow rate is different from in example 2, byappropriately adjusting the purge gas flow rate so that the growth rateon the face tilted from the c-face is less than one-tenth of the growthrate on the c-face, a similar result to that of example 2 is provided.

In addition, the method of example 4 has the advantage of reducing theamount of hydrogen contained in the gas to be fed as a raw material gasmore than in the case of examples 1 to 3. For this reason, the method ofexample 4 has the advantage of reducing the etching action in theintended growth face more than those of examples 1 to 3, thereforeenhancing material efficiency.

Example 5

In example 5, on the tray 3 of the HVPE apparatus shown in FIG. 7, a 3mm high cylindrical metallic ring is added and installed, and the innersurface of the ring and the installation surface 4 of the tray 3 areconfigured as the inner surface of the container in the shape of thecrucible. As the material of the above ring, Ti (titanium) is used, andis nitrided in 2 slm ammonia and 8 slm hydrogen in the HVPE apparatusfor 2 hours prior to growth, to change the surface of the ring into atitanium nitride and crystal growth is performed.

With the HVPE apparatus added with this ring, a similar experiment tothat of comparative example 1 are performed. Advantageous effects ofenhanced yield, improved crystallinity and increased curvature radiussimilar to those of example 1, are provided. Further, in this case,etching of the face tilted from the c-face occurs, and the etching rateis about one third of the growth rate on the c-face.

In example 5, there is no purge gas feed to the epitaxial waferperimeter, but the same advantageous effect as that of the purge gasfeed is provided. This is considered to be because the metal nitride ofthe ring that makes up the sidewall of the container is a catalyst, andhydrogen contained in the raw material gas is decomposed to produceatomic hydrogen with a strong etching action.

When the same sidewall is configured as for example a quartz or carbonring, cracking is not suppressed. Further, even in the case of using aring of metal nitride, when the total flow rate of hydrogen in the rawmaterial gas is 1 slm or less, the growth rate on the face tilted fromthe c-face is faster, and cracking occurs. From the above results, it isshown that the presence of the metal nitride and some amount of hydrogenare necessary, and the idea of the development of the etching action dueto the occurrence of the atomic hydrogen due to the catalyst action ofthe metal nitride is supported.

Further, in the case of changing the hydrogen flow rate in the rawmaterial gas in the range of 2 to 7 slm, as in the case of changing theflow rate of the purge gas in example 2, the growth rate (etching rate)on the face tilted from the c-face is changed, but the growth rate isless than one-tenth of the growth rate on the c-face, while the etchingrate is not more than the growth rate on the c-face, and in this range,advantageous effects of enhanced yield, and improved crystallinitysimilar to those of example 1, are observed.

Example 6

In example 6, an experiment similar to example 5 is performed bychanging the metal material of the ring constituting the containersidewall into any of Zr, Nb, Ta, Cr, W, Mo, and Ni. As a result, similarresults to those of example 5 are provided.

Example 7

In example 7, the same experiment as in examples 1 to 6 is performed byintroducing dichlorosilane (SiH₂Cl₂) from the etching/doping line 24during a growth of 2 to 3 μm at a top of the GaN layer, to grow an n-GaNlayer with an impurity concentration of 0.5×10¹⁸/cm³ to 5×10¹⁸/cm³. Thegrowth uses both a background impurity concentration (impurityconcentration in undoping) varying depending on growth conditions, anddichlorosilane doping. Also in this case, similar results to those ofexamples 1 to 6 are provided.

Example 8

In example 8, a similar experiment to in examples 1 to 7 is performed byvariously changing the growth temperature, gas flow rate, growth rate,and growth pressure. Although the resulting yield and the XRD full widthat half maximum is slightly different from those of the examples above,when the growth rate of the face tilted from the c-face of the outerportion of the GaN crystal is less than one-tenth of the growth rate ofthe c-face, advantageous effects of enhanced yield, and improvedcrystallinity similar to those of examples 1 to 7, are provided.

In addition, throughout the experiments to date, the angle between theGa-polar c-face of the GaN growth layer and the face tilted from thec-face occurring around the outer portion tends to be 90 degrees in theend facing in the m axis direction of the GaN crystal and formed withthe m-face, and around the end other than it, as shown in FIG. 12, aface tilted at an angle between the c-face and the face tilted from thec-face of 110 degrees to 135 degrees occurs.

Example 9

In example 9, an experiment similar to those of examples 1 to 8 isperformed by changing the distance between the GaN crystal and thesidewall in the range of 0.5 to 20 mm. When the above distance issmaller than 1 mm, during the growth, the GaN crystal of the end face ofthe epitaxial wafer may come into contact with the container sidewall.In this case, due to the GaN crystal of the epitaxial wafer end face andthe sidewall being fixed, stress occurs during growth, and crackingtends to occur. Further, when the above distance is greater than 10 mm,the local application of the etching action to only the end of theepitaxial wafer is difficult, and when such conditions as not to causecracking are selected, a significant reduction of the region to grow theGaN layer is seen.

From the above, it is deemed appropriate that the distance between theGaN crystal and the sidewall is from 1 to 10 mm.

From the examples described above, it is concluded that in order to growthe GaN layer which is good in the yield, thick and high in thecrystallinity (narrow in the XRD full width at half maximum), it isimportant to install the seed crystal substrate in the crucible shapedcontainer having the sidewall, and maintain the distance between thesidewall with the etching action weakening with distance and theperimeter of the GaN crystal in the range of 1 to 10 mm, i.e., to growthe GaN crystal with the shape substantially similar to the shape of theinner surface of the crucible shaped container, in no contact with theportion of the inner surface of the container, which is not contactedwith GaN at growth initiation, throughout the entire growth period. Inaddition, as a feature of the GaN layer realized by the above-describedexamples, it is particularly notable that the growth thickness of thecrystal portion with the high impurity concentration lying in the outerportion of the crystal on the deliberately grown face (in the examplesmainly the Ga-polar c-face) is not more than one-tenth of the growththickness of the crystal on the deliberately grown face. Also, it is animportant advantage in device application that the GaN epitaxial waferin the above-described embodiments has the radius of curvature largerthan that of the conventional method.

Example 10 and Comparative Example 2

In example 10 and comparative example 2, a GaN freestanding substrate isproduced by the void-assisted separation (VAS) method described inJP-A-2004-039810 described above.

Comparative Example 2

In FIG. 13, there is shown a schematic of the VAS method. A voidsubstrate 40 is first prepared as a seed crystal substrate (FIG. 13A).The void substrate 40 results from a metal organic vapor phase epitaxy(MOVPE) method or the like growing an about 300 nm thick GaN thin filmover a sapphire substrate 41, depositing a Ti film over its surface, andthen heat treating it in hydrogen and ammonia. The void substrate 40results from the above-described heat treatment converting the Ti filminto a mesh structure TiN layer 43, and forming a multiplicity of voids44 in the GaN thin film to produce a void containing GaN layer 43.

Then, over the void substrate 40, a thick GaN layer 45 is grown by theHVPE method (FIG. 13B), and then the sapphire substrate 41 is separatedfrom the void portion. This results in a GaN crystal (GaN singlecrystal) 46, which is a GaN freestanding substrate (FIG. 13C).

As the sapphire substrate 41, there is used one having a surface tiltedin the range of 0.05 to 2 degrees in the a-axis or m-axis direction fromthe c-face or in a direction therebetween, and having a thickness of 300to 1500 μm, and a diameter of 35 to 200 mm. The thickness of Ti duringthe above-mentioned void substrate production is 5 to 100 nm.

Over the void substrate 40, the GaN single crystal having a diameter of35 to 200 mm and a thickness of 50 μm to 10 mm is produced. The HVPEgrowth conditions are as follows: For example, the substrate temperatureis 800 to 1200 degrees Celsius, the pressure is 10 kPa to 120 kPa, andthe growth rate is 30 to 1000 μm/h. As the growth apparatus, there isused the HVPE apparatus shown in FIG. 7. The flow rates of each line areset in the following ranges. From the group III line 25, 25 to 1000 ccmof HCl and 2 slm of hydrogen are added, and the flow rate of nitrogen isset such that the total flow rate of the group III line 25 is 3 slm.From the group V line 23, 1 to 2 slm of ammonia and 1 slm hydrogen areadded, and the flow rate of nitrogen is set such that the total flowrate of the group V line 23 is 3 slm. In addition, from theetching/doping line 24, 3 slm hydrogen is caused to flow.

The dislocation density of the GaN single crystal formed over the voidsubstrate is determined by the thickness of the Ti film during voidsubstrate fabrication. As the Ti film is thinner, the dislocation in thevoid containing GaN layer 43 MOVPE grown of the void substrate tends topropagate to the thick GaN layer 45 formed over the Ti film, andtherefore the dislocation density is high. The dislocation density ofthe GaN single crystal to provide the Ti film thickness in the range of5 to 100 nm is in the range of 1×10⁴/cm² to 1×10⁸/cm².

Further, any surfaces of the resulting GaN crystals are mirror surfaceshaving few pits after growth. The electron density in the GaN crystal isset in the range of 1×10¹⁵/cm³ to 5×10¹⁸/cm³, by adjusting the flow rateof dichlorosilane to be added during undoped growth or growth.

When growing GaN crystals (GaN layers) having a thickness of 50 μm to 10mm by the combination of these various conditions, the dependence of theyield (defined as the ratio of being not poor when growing 20 wafers inthe same conditions, and as poor when a crack with a length of 5 mm ormore occurs) on the growth thickness is indicated by a circle o in FIG.14. The yield is almost not dependent on the carrier concentration andthe dislocation density, but is strongly dependent on only the thicknessof the GaN crystal. When the thickness of the GaN crystal is not morethan 100 μm, the resulting yield is nearly 100%, but when the thicknessof the GaN crystal exceeds 100 μm, the yield decreases rapidly, and theyield of the GaN freestanding substrate (the GaN crystal) having athickness exceeding 800 μm is less than 10%.

The cross sections of these GaN freestanding substrates (GaN crystals)are observed by a fluorescence microscopy. As shown in FIG. 9, differentcolor regions similar to those of comparative example 1 are observedaround the end of the GaN freestanding substrates, and the growth ratein a direction perpendicular to the side surface is not less than 1/10of the growth rate perpendicular to the principal surface. These GaNcrystals in each portion are examined by the micro Raman method. The GaNcrystal grown on the tilted face of the end of the freestandingsubstrates (GaN crystals), and the GaN crystal grown on the Ga polarc-face are different in impurity concentration: In the GaN crystal onthe c-face, the impurity concentration is of n-type and is about0.5×10¹⁸/cm³ to 5×10¹⁸/cm³, while in the GaN crystal grown on the facetilted from the c-face, the impurity concentration is of the samen-type, but is as very high as 1×10¹⁹/cm³ to 5×10¹⁹/cm³ which is twotimes or more the impurity concentration in the GaN crystal on thec-face.

Example 10

On the other hand, in example 10, in the same manner as in example 5,there is used the HVPE apparatus in which the ring of metal nitrideserving as the catalyst is installed on the tray 3 in FIG. 7, or theHVPE apparatus shown in FIG. 10. In the same manner as examples 1 to 9,with the method of the present invention to introduce the diluting gas,etching gas or hydrogen gas around the perimeter of the void substrate40 which is the seed crystal substrate, and in the same manner as inexamples 1 to 9, when the growth rate of the GaN crystal of the facetilted from the c-face is set at less than one-tenth of the growth rateof the GaN crystal on the c-face, the yield is enhanced dramatically asindicated by an x-mark x in FIG. 14. Until the thickness of the GaNlayer is 1500 μm, the resulting yield is almost 100%, and is maintainedat 10% even in the thickest case of 10 mm.

However, the above results are limited to only the case the surface ofthe GaN layer 45 is located to be lower than the height of the containersidewall. When the growth surface of the GaN layer 45 is higher than theheight of the sidewall, the yield drops sharply. This is because if thesurface of the GaN layer 45 is higher than the sidewall, the etchingaction in the outer portion of the GaN crystal is weakened, growtharound the outer portion occurs, and therefore stress occurs. Also, ifthe distance between the GaN crystal 45 and the sidewall is less than 1mm, the sidewall and the GaN crystal are fixed, cracking occurs, and theyield worsens. When the distance between the GaN crystal 45 and thesidewall is wider than 10 mm, no decrease in the yield occurs, but it isdifficult to localize the etching action in only the end face, and thegrowth region of the GaN crystal is greatly reduced.

Further, as described in example 8, the angle between the Ga-polarc-face of the GaN growth layer and the face tilted from the c-faceoccurring around the outer portion tends to be 90 degrees in the endfacing in the M axis direction of the GaN crystal and formed with them-face, and around the end other than it, as shown in FIG. 12, a facetilted at an angle between the c-face and the face tilted from thec-face of 110 degrees to 135 degrees tends to occur. However, byappropriately selecting the growth conditions, it is also possible toset this angle at 90 degrees around the entire perimeter.

From Example 10 and comparative example 2, it is concluded that in orderto produce the GaN freestanding substrate with good yield and greatthickness, it is important to install the seed crystal substrate in thecrucible shaped container having the sidewall, and maintain the distancebetween the sidewall with the etching action weakening with distance andthe perimeter of the substrate in the range of 1 to 10 mm, i.e., to growthe GaN crystal with the shape substantially similar to the shape of theinner surface of the crucible shaped container, in no contact with theportion of the inner surface of the container, which is not contactedwith the seed crystal substrate at growth initiation, throughout theentire growth period. In addition, as a feature of the GaN freestandingsubstrate realized by this example, it is particularly notable that inthe outer portion of the GaN freestanding substrate, the growththickness of the crystal portion with the higher impurity concentrationthan that of the crystal on the c-face is not more than one-tenth of thegrowth thickness of the crystal on the c-face.

Example 11 and Comparative Example 3

In general, in GaN freestanding substrates realized on heterogeneoussubstrates by various methods including the VAS method, a growing GaNcrystal has a significant reduction in the dislocation density of forexample from 10⁸/cm² to 10⁵/cm² caused in the thickness direction.Because residual strain is introduced in the GaN crystal due to thisreduction in the dislocation density, there in many cases remains strainin the GaN freestanding substrates grown over the heterogeneoussubstrates. Typically, in the case of the GaN freestanding substrates inwhich the c-face is the surface thereof, the c-face has a curvatureradius of 2 to 4 m immediately after growth.

The presence of this strain leads to the yield lowering due to theoccurrence of cracking when the GaN Growth thickness exceeds 1500 μm,even in the case of using the method of the present invention, as shownin FIGS. 8A to 8C of example 10.

Such residual strain can be greatly reduced by polishing and removingthe back side of the GaN freestanding substrate produced by the VASmethod or the like. For example, if after growth of the 1500 μm thickGaN freestanding substrate, its back side is removed by 1000 μm, thecurvature radius of the c-face which is 2 to 4 m immediately after thegrowth increases to 10 m or more. If the back side polished GaNfreestanding substrate is used as the seed crystal, since the residualstress becomes very small, it can be expected that it is possible togrow the GaN layer which is impossibly thick in the growth over theheterogeneous substrate.

Comparative Example 3

In comparative example 3, with the method of the present invention inExample 10, the back side (the N polar c-face) of the GaN freestandingsubstrate having a thickness of 1500 μm grown by the VAS method ispolished by 1000 μm, and further the front side (the Ga polar c-face) ispolished by 100 μm, resulting in the GaN freestanding substrate having athickness of 400 μm with enhanced flatness being used as the seedcrystal. And, over the Ga-polar front side of this GaN freestandingsubstrate is grown the GaN layer by HVPE.

As the GaN substrate which is the seed crystal, there is used one havinga surface tilted in a range of 0.05 to 2 degrees in the a-axis or m-axisdirection from the Ga-polar c-face or in a direction therebetween, andhaving a thickness of 400 mm, and a diameter of 35 to 200 mm. Thetypical dislocation density is 1×10⁶/cm².

The GaN single crystals having a thickness of 50 μm to 100 mm areproduced. The HVPE growth conditions are as follows: For example, thesubstrate temperature is 800 to 1200 degrees Celsius, the pressure is 10kPa to 120 kPa, and the growth rate is 30 to 1000 μm/h. As the growthapparatus, there is used the HVPE apparatus shown in FIG. 7. The flowrates of each line are set in the following ranges. From the group IIIline 25, 25 to 1000 ccm of HCl and 2 slm of hydrogen are added, and theflow rate of nitrogen is set such that the total flow rate of the groupIII line 25 is 3 slm. From the group V line 23, 1 to 2 slm of ammoniaand 1 slm hydrogen are added, and the flow rate of nitrogen is set suchthat the total flow rate of the group V line 23 is 3 slm. In addition,from the etching/doping line 24, 3 slm hydrogen is caused to flow.

Also, any surfaces of the GaN single crystals (seed crystals) used hereare mirror surfaces having few pits after growth. For the electrondensity in the GaN crystals, the GaN crystals are prepared which havethe electron density in the range of 1×10¹⁵/cm³ to 5×10¹⁸/cm³, byadjusting the flow rate of dichlorosilane to be added during undopedgrowth or growth.

When growing GaN crystals (GaN layers) having a thickness of 50 μm to100 mm by the combination of these various conditions, the dependence ofthe yield (defined as poor when a crack with a length of 5 mm or moreoccurs) on the growth thickness is indicated by a circle o in FIG. 15.Even in this case, the yield is almost not dependent on the carrierconcentration and the dislocation density, but is strongly dependent ononly the thickness of the GaN crystal. When the thickness of the GaNcrystal newly grown over the GaN crystal (seed crystal substrate) by theVAS method is not more than 500 μm, the resulting yield is nearly 100%,but when the thickness of the growth layer of the GaN crystal exceeds 1mm, the yield decreases rapidly, and the yield of the GaN freestandingsubstrate (the GaN crystal) having a thickness exceeding 10 mm is lessthan 10%.

The cross sections of these GaN freestanding substrates are observed bya fluorescence microscopy. As schematically shown in FIG. 2, differentcolor regions similar to those of FIG. 9 are observed around the end ofthe GaN freestanding substrates, and the growth rates in a directionperpendicular to respective faces thereof are substantially the same.Each portion of these is examined by the micro Raman method. The crystalgrown on the tilted face of the wafer end, and the crystal grown on theGa polar c-face are different in impurity concentration: In the crystalon the c-face, the impurity concentration is of n-type and is about0.5×10¹⁸/cm³ to 5×10¹⁸/cm³, while in the crystal grown on the facetilted from the c-face, the impurity concentration is of the samen-type, but is as very high as 1×10¹⁹/cm³ to 5×10¹⁹/cm³ which is twotimes or more the impurity concentration in the GaN crystal on thec-face.

Example 11

On the other hand, in example 11, in the same manner as in example 5,there is used the HVPE apparatus in which the ring of metal nitrideserving as the catalyst is installed on the tray 3 in FIG. 7, or theHVPE apparatus shown in FIG. 10. In the same manner as examples 1 to 9,the GaN crystal is grown by the method of the present invention tointroduce the diluting gas, etching gas or hydrogen gas around theperimeter of the GaN substrate which is the seed crystal substrate. Inthe same manner as in examples 1 to 9, when the growth rate of the GaNcrystal of the face tilted from the c-face is set at less than one-tenthof the growth rate of the GaN crystal on the intended growth face, theyield is enhanced dramatically as indicated by an x-mark x in FIG. 15.Until the thickness of the GaN layer newly grown over the GaN crystal(seed crystal substrate) by the VAS method is 5 mm, the yield ismaintained at almost 100%, and is still maintained at 50% even when thethickness of the grown layer of this GaN crystal is 100 mm.

In FIG. 16, there is schematically shown a result of fluorescencemicroscope observation of a cross-section of the outer end of the GaNfreestanding substrate grown as the seed crystal substrate using example11. Although the crystal 2 b of the high impurity concentration of theend face is present, its amount is small, and the growth rate of thecrystal 2 b is estimated to be, at maximum, less than one-tenth of thegrowth rate of the crystal 2 a on the c-face. Further, when the growththickness of the GaN crystal 2 is thin, since a crystal face f₂ istilted from the c-face around the end with crystal growth, the area ofthe c-face f₁ is reduced gradually. However, the c-face f₁ does notcontinue to contract, and the GaN crystal 2 is grown to be thick (about2 mm at maximum) to some extent. After the distance between the end faceand the sidewall is a value, the growth progresses with the end faceforming a perpendicular face f₃ at 90 degrees to the c-face and with theshape and area of the c-face being held constant. This is a phenomenonthat results from the distance between the end face and the innersurface of the container sidewall being widened, the etching action fromthe sidewall being weakened, and the growth and the etching on the endface being balanced.

However, the above results are limited to only the case the surface ofthe GaN crystal 2 a grown on the c-face is located to be lower than theheight of the container sidewall. When the growth surface of the GaNcrystal 2 a is higher than the height of the sidewall, the yield dropssharply. This is because if the surface of the GaN crystal 2 a is higherthan the sidewall, the etching action in the outer portion of the GaNcrystal 2 a is weakened, crystal growth around the outer portion occurs,and therefore stress occurs. Also, if the distance between the GaNcrystal 2 a and the sidewall is less than 1 mm, the sidewall and the GaNcrystal 2 a are fixed, cracking occurs, and the yield worsens. When thedistance between the GaN crystal 2 a and the sidewall is wider than 10mm, no decrease in the yield occurs, but it is difficult to localize theetching action in only the end face, and the growth region of the GaNcrystal is greatly reduced.

From Example 11 and Comparative Example 3, it is concluded that in orderto produce the GaN freestanding substrate with good yield and greatthickness, even in the case of using the nitride semiconductorfreestanding substrate as the seed crystal, it is important to installthe seed crystal substrate in the crucible shaped container having thesidewall, and maintain the distance between the sidewall with theetching action weakening with distance and the perimeter of the GaNcrystal in the range of 1 to 10 mm, i.e., to grow the GaN crystal withthe shape substantially similar to the shape of the inner surface of thecrucible shaped container, in no contact with the portion of the innersurface of the container, which is not contacted with the seed crystalsubstrate at growth initiation, throughout the entire growth period. Inaddition, as a feature of the GaN freestanding substrate realized bythis example, it is particularly notable that in the outer portion ofthe GaN freestanding substrate, the growth thickness of the crystalportion with the higher impurity concentration than that of the crystalon the c-face is not more than one-tenth of the growth thickness of thecrystal on the c-face.

Example 12

In example 11, when the GaN growth thickness exceeds 5 mm, the yield isdecreased due to slight strain remaining in the GaN freestandingsubstrate used as the seed crystal substrate. Therefore, in example 12,further enhancement of the yield is attempted by applying the etchingaction not only to the outer end face of the GaN crystal, but also tothe back side of the GaN freestanding substrate with residual strainused as the seed crystal substrate during growth.

For a method used in example 12, in a manner similar to example 11, ablock (block 17 as shown in FIG. 6 of the above-described embodiment)having a height of 1 to 2 mm and made of quartz, carbon, or metalnitride is installed on the back side of the seed crystal substrate, andthe seed crystal substrate is floated from the installation surface (forexample, the bottom surface of the container), to also apply the etchingaction to the back side of the seed crystal substrate. For example,there is used the method by also providing an outlet for the purge gasat the center of the installation surface, and introducing therefrom anetching gas such as hydrogen, chlorine, hydrogen chloride or the like,or the method by forming the installation surface from the metalnitride, and adding hydrogen to the raw material gas.

Although the etching rate of the back side of the seed crystal substrateby these methods changes due to temperature, growth atmosphere, growthpressure, purge gas flow rate, etc., the higher yield than that ofexample 11 can be provided when the etching rate is not less thanone-hundredth of the growth rate on the Ga-polar c-face. When theetching rate of the back side is increased, the yield is increased, butwhen the etching rate of the back side is not less than one-twentieth ofthe growth rate on the Ga-polar c-face, the yield is constant.

In FIG. 17, there is shown the relationship between the yield and thethickness of the GaN growth layer when the etching rate of the back sideis one-twentieth of the growth rate on the Ga-polar c-face. The yield ashigh as 90% is still maintained even when the thickness of the GaNgrowth layer is 100 mm.

Example 13

In example 13, an experiment similar to those of examples 11 and 12 isperformed by using not the Ga polar c-face, but the N polar c-face asthe growth face, to grow GaN, to produce the GaN freestanding substrate.

In this case, by the use of the conventional HVPE apparatus, the endface of the GaN crystal has an opposite inclination to that of FIG. 16,and the c-face is expanded with growth. For this reason, the growth ofthe GaN freestanding substrate by the N polar face growth is the veryeffective method in order to realize the larger-diameter GaN substrate.

However, in the case of using the conventional HVPE apparatus, as wellas in the Ga polar face growth, in the N polar face growth, stress iscaused by the crystal growth in the end face, and it is difficult toprovide the high yield.

However, by using the method of the present invention, it is confirmedthat even in the N polar face growth, it is possible to fabricate theGaN freestanding substrate with high yield as in examples 11 and 12.Furthermore, in this case, by the expansion tendency of the N polar faceand the etching action being balanced, it is possible to produce thecylindrical freestanding substrate with the GaN crystal grown to beconstant in the area of its N polar face similar to the cylindricalcontainer, shape. This freestanding substrate is industrially veryuseful in that wafers having a constant diameter can be producedefficiently by slicing this freestanding substrate.

Also, in particular, in the N polar face growth, for example, as in theabove embodiment shown in FIG. 5, by setting the angle θ between theside surface of the sidewall 12 a of the container 12 and the placementsurface 15 of the bottom wall 12 b of the container 12 to place the seedcrystal substrate 1 in the range of greater than 90 degrees and notgreater than 135 degrees, and making the side surface of the sidewall 12a open toward the opening, it is possible to expand the area of the Npolar face more than the seed crystal substrate 1, while applying theetching action to the end of the GaN crystal 2.

If the angle θ between the side surface of the container 12 and theplacement surface 15 of the container 12 is greater than 135 degrees,since the crystal face which tends to appear at the end of the GaNcrystal has an angle of not greater than 135 degrees, the distancebetween the side surface of the container 12 and the outer end of theGaN crystal 2 is increased with the growth of the GaN layer, and GaNgrowth occurs at the outer end of the GaN crystal 2, and cracking tendsto occur due to this GaN growth. Only a same result as that of theconventional method can be provided.

When this angle θ is not greater than 135 degrees, since the distancebetween the side surface of the container 12 and the outer end of theGaN crystal 2 tends to be held constant to balance the growth and theetching to the end of the GaN crystal 2, the growth rate of the outerend of the GaN crystal 2 is held at substantially zero, to suppress theoccurrence of cracking. In particular, when the angle θ is not greaterthan 120 degrees, since the stable GaN crystal faces having an anglesmaller than this are lessened, a higher growth yield can be achievedand substantially the same result as the results of the above examplesshown in FIGS. 14 and 17 is provided.

Next, modifications of the present invention are described below.

(Modification 1)

In modification 1, an experiment similar to examples 1 to 9 is performedby setting the diameter of the sapphire substrate at 50 to 200 mm, andtaking the surface (principal surface) of the sapphire substrate as aface tilted in a range of 0.1 to 2 degrees in the a-axis or m-axisdirection or a direction therebetween from the Ga-polar c-face, or ana-face, m-face, r-face or other semi-polar faces, or slightly tiltedfaces thereof, etc. A substantially same result as those of examples 1to 9 is provided.

(Modification 2)

In modification 2, a similar experiment to modification 1 is performedby changing the sapphire substrate to a SiC substrate, and a Sisubstrate. A similar advantageous effect to that of modification 1 isconfirmed.

(Modification 3)

In modification 3, the same experiment as in examples 1 to 9 isperformed by changing the buffer layer from the low temperature grownGaN buffer layer to a low-temperature grown AlN buffer layer and a hightemperature grown AlN buffer layer. Each buffer layer thickness isbetween 10 nm to 2 μm. In any case, a result similar to those ofexamples 1 to 9 is provided.

(Modification 4)

In modification 4, an experiment similar to that of examples 1 to 9 isperformed by using the seed crystal substrates with irregularities in anupper surface thereof. For the shape of the irregularities, the heightof the protrusions is 0.1 to 2 μm, the pitch is 1 to 10 μm, and thereare used a bowl shape, conical shape, polygonal pyramid shapes oftriangular pyramid shape to hexagonal pyramid shape, and these shapeshaving a flat portion on top thereof. Also, for the arrangement of theprotrusions, the protrusions are arranged in a triangular or squaregrid, and the arrangement is used such that a side of the grid is in thea-axis or the m-axis direction. Forming a light-emitting element byusing an underlying nitride semiconductor layer using those seed crystalsubstrates with the irregularities allows for enhancing light extractionefficiency more than the light-emitting element when used over the flatseed crystal substrate.

In any case of modification 4, when applying the nitride semiconductorcrystal producing method of the present invention, a same advantageouseffect as those of examples 1 to 9 is provided.

(Modification 5)

In modification 5, an experiment similar to examples 10 to 13 isperformed by setting an a-face, m-face, r-face or other semi-polarfaces, or slightly tilted faces thereof, etc. A substantially sameresult as those of examples 10 to 13 is provided.

(Modification 6)

For the principle of the nitride semiconductor crystal producing methodof the present invention, the growth method is not limited to the HVPEmethod, but may also be applied to the case of changing the growthmethod to the MOVPE method, the ammonothermal synthesis method, or theNa flux method.

(Modification 7)

The nitride semiconductor crystal producing method of the presentinvention may also be applied to nitride semiconductor materials otherthan GaN, such as MN, InN, and BN, and a mixed crystal of thesematerials including GaN.

(Modification 8)

The principle of the nitride semiconductor crystal producing method ofthe present invention may be also applied to semiconductors other thanthe nitride semiconductors or to crystalline materials other than thesemiconductors.

(Modification 9)

The method of the present invention may be applied not only to thevertical arrangement crystal growing apparatuses (HVPE apparatus, MOVPEapparatus) as shown in FIG. 7 or FIG. 10, but also to a horizontal flowarrangement MOVPE apparatus, or HVPE apparatus shown in FIG. 18. Thatis, as shown in FIG. 18, a rectangular tube reactor (growth furnace) 50is horizontally arranged, and an opening of a bottom wall of the reactor50 is provided with a container 51 having a sidewall 51 a, and in thecontainer 51 the tray 3 is installed at a distance from the bottom wall51 b of the container 51. The bottom wall 51 b is connected with a feedpipe 53 to feed a gas g having the etching action, and a rotary shaft 52is provided to be passed through the feed pipe 53. The tray 3 isrotatably supported on the rotary shaft 52, and an outer portion of thereactor 50 is provided with a peak (not shown). A raw material gas Gflows horizontally from one end to the other end in the reactor 50, togrow a crystal over the seed crystal substrate 1 installed on the tray3. On the other hand, a gas g fed into the container 51 from the feedpipe 52 flows radially along the tray 3 between the tray 3 and thebottom wall 10 b, and flows out from a gap between the outer surface ofthe tray 3 and the inner surface of the sidewall 10 a.

Also, the method of the present invention may also be applied to, forexample, a center-outlet self-revolution multi-charge crystal growthapparatus, i.e., a crystal growth apparatus in which a plurality of seedcrystal substrates are disposed along the same circumference on asusceptor, and the plural seed crystal substrates on the susceptor areself-revolved, to cause the raw material gas to flow from the center ofthe susceptor, along the susceptor, radially, to each seed crystalsubstrate.

Further, by devising a way of holding the seed crystal substrate, themethod of the present invention may also be applied not only to theface-up crystal growth apparatus in which the growth face faces up as inFIG. 7, FIG. 10, or FIG. 18, but also to a face-down crystal growthapparatus in which the growth face faces down, or a crystal growthapparatus in which the growth face faces in a vertical direction or anobliquely tilted direction. However, when etching the back side of theseed crystal substrate, it is necessary to reduce the etched amount ofthe back side to a certain amount, because if the seed crystal substrateis all etched, the crystal shifts or falls off the position at theinitiation of growth.

Although the invention has been described with respect to the specificembodiments and modifications thereof, the appended claims are not to bethus limited to the specific embodiments and the modifications. Itshould be noted that all of the combinations of features described inthe specific embodiments and the modifications are not necessarilyessential for the means for solving the problems of the Invention.

What is claimed is:
 1. A nitride semiconductor crystal producing method,comprising growing a nitride semiconductor crystal over a seed crystalsubstrate, while applying an etching action to an outer end of the seedcrystal substrate during the growing of the nitride semiconductorcrystal.
 2. The nitride semiconductor crystal producing method accordingto claim 1, further comprising installing the seed crystal substrate ina container with an inner surface and a sidewall for surrounding anouter side of the seed crystal substrate, wherein an environmentadjacent to a portion of the inner surface of the container, which isnot contacted with the seed crystal substrate at growth initiation, isan environment to apply an etching action during the nitridesemiconductor crystal growing, so that the nitride semiconductor crystalis grown in no contact with the portion of the inner surface of thecontainer throughout an entire period of crystal growth, and in across-sectional shape similar to an inner cross-sectional shape of thecontainer.
 3. The nitride semiconductor crystal producing methodaccording to claim 2, wherein the inner surface of the container, whichis not contacted with the nitride semiconductor crystal, includes a sidesurface of the sidewall.
 4. The nitride semiconductor crystal producingmethod according to claim 2, wherein the inner surface of the container,which is not contacted with the nitride semiconductor crystal, includesa surface on a side in which the seed crystal substrate is installed. 5.The nitride semiconductor crystal producing method according to claim 2,wherein the environment adjacent to the portion, of the inner surface ofthe container, which is not contacted with the nitride semiconductorcrystal, is an environment in which the etching action weakens withdistance from a side surface of the sidewall.
 6. The nitridesemiconductor crystal producing method according to claim 2, wherein thenitride semiconductor crystal is grown in an environment of coexistenceof growth and etching, and the etching action is strengthened bydiluting a growing raw material adjacent to the portion of the innersurface of the container, which is not contacted with the nitridesemiconductor crystal.
 7. The nitride semiconductor crystal producingmethod according to claim 6, wherein the growing raw material is dilutedby feeding an inert gas containing nitrogen, argon or helium.
 8. Thenitride semiconductor crystal producing method according to claim 2,wherein the etching action is applied by feeding an etching gas orliquid adjacent to the portion of the inner surface of the container,which is not contacted with the nitride semiconductor crystal.
 9. Thenitride semiconductor crystal producing method according to claim 8,wherein the etching gas contains at least any one of hydrogen, chlorine,and hydrogen chloride.
 10. The nitride semiconductor crystal producingmethod according to claim 2, wherein the nitride semiconductor crystalis grown by feeding a substance for producing an etching species througha catalytic action, and at least a portion of the inner surface of thecontainer, which is not contacted with the nitride semiconductorcrystal, comprises a catalyst having the catalytic action, to therebydevelop the etching action.
 11. The nitride semiconductor crystalproducing method according to claim 10, wherein the substance forproducing the etching species through the catalytic action is a hydrogengas.
 12. The nitride semiconductor crystal producing method according toclaim 10, wherein the catalyst having the catalytic action is a metal ora metal nitride.
 13. The nitride semiconductor crystal producing methodaccording to claim 12, wherein the metal is any of Ti, Zr, Nb, Ta, Cr,W, Mo, or Ni.
 14. The nitride semiconductor crystal producing methodaccording to claim 2, wherein a distance between the etchingaction-producing inner surface of the container, which is not contactedwith the nitride semiconductor crystal, and the nitride semiconductorcrystal, ranges from 1 to 10 mm, for a period from crystal growthinitiation until termination.
 15. The nitride semiconductor crystalproducing method according to claim 2, wherein an angle between a sidesurface of the sidewall and a placement surface of the container onwhich the seed crystal substrate is placed ranges greater than 90degrees and not greater than 135 degrees, so that the inner crosssection of the container is shaped to expand toward its opening side,and the nitride semiconductor crystal is grown having a nitrogen face asa growth face, while expanding its diameter.
 16. A nitride semiconductorepitaxial wafer, comprising: a plate-like seed crystal substrate; and anitride semiconductor crystal grown over the seed crystal substrate,wherein the nitride semiconductor crystal includes a nitridesemiconductor crystal grown in a principal surface direction of the seedcrystal substrate, but around an outer end of the nitride semiconductorcrystal grown in the principal surface direction, there is included nonitride semiconductor crystal grown in a face direction tilted from theprincipal surface, and having a higher impurity concentration than thenitride semiconductor crystal grown in the principal surface direction,or even when there is included the nitride semiconductor crystal havingthe higher impurity concentration around the outer end, a growththickness of the nitride semiconductor crystal having the higherimpurity concentration around the outer end is less than one tenth of agrowth thickness of the nitride semiconductor crystal grown in theprincipal surface direction.
 17. The nitride semiconductor epitaxialwafer according to claim 16, wherein the seed crystal substrate is asapphire substrate, and the nitride semiconductor crystal is a GaNlayer, and let a curvature radius of the nitride semiconductor epitaxialwafer be R (m), a thickness of the GaN layer be t (μm), a thickness ofthe sapphire substrate be Y (μm), and a coefficient be A, then thefollowing relations (1) and (2) are satisfied.R=A/t  (1)A>0.00249×Y ^(1.58483)  (2)
 18. A plate-like nitride semiconductorfreestanding substrate, comprising a nitride semiconductor crystal grownin a principal surface direction of the nitride semiconductorfreestanding substrate, wherein around an outer end of the nitridesemiconductor freestanding substrate, there is included no nitridesemiconductor crystal having a higher impurity concentration than thenitride semiconductor crystal grown in the principal surface direction,or even when there is included the nitride semiconductor crystal havingthe higher impurity concentration around the outer end, a growththickness of the nitride semiconductor crystal having the higherimpurity concentration around the outer end is less than one tenth of agrowth thickness of the nitride semiconductor crystal grown in theprincipal surface direction.